Type-juggling and (strict) greater/lesser-than comparisons in PHP
PHP is famous for its type-juggling. I must admit it puzzles me, and I\'m having a hard time to find out basic logical/fundamental things in comparisons.
For example
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There are no strict identical comparison operators (
>==or<==) in PHP (by PHP 5.6.14 at least), but there are a few ways to enforce a strict type check before checking Greater/Lower:- Check both variable types with
if (gettype($a) === gettype($b)) - Force your needed type-cast eg.
if ((string)$a === (string)$b) - Force your needed type-juggle eg.
if (($a . '') === ($b . ''))
Take note that:
- Floating point precision is limited
INFandNANare of typefloatunder ieee754- Some Infinity equals some other Infinity (since PHP 5.4)
- Scientific notation
eis always of typefloat, and neverintegereven if the number is small - Integers going over
PHP_INT_MAXget automatically converted tofloat - Floats over system's boundaries get the
INFvalue - Undefined variables are of type and value
NULL - Integers preceded by
0are converted from octal to decimal (by convention) - Converting Strings containing an integer with a leading
0to integer strips the leading0
List of some exotic comparisons:
Very strange: $a VS. $b $a>$b $a<$b $a<=$b $a>=$b $a==$b $a===$b float(NAN) float(-INF) false false false false false false float(NAN) float(0) false false false false false false float(NAN) float(1) false false false false false false float(NAN) float(INF) false false false false false false float(NAN) float(NAN) false false false false false false float(NAN) int(-1) false false false false false false float(NAN) int(0) false false false false false false float(NAN) int(1) false false false false false falseEqual but not identical:
$a VS. $b $a>$b $a<$b $a<=$b $a>=$b $a==$b $a===$bNULL(NULL) array() false false true true true false NULL(NULL) bool(false) false false true true true false NULL(NULL) float(0) false false true true true false NULL(NULL) int(0) false false true true true false NULL(NULL) str('') false false true true true false array() bool(false) false false true true true false bool(false) float(0) false false true true true false bool(false) int(0) false false true true true false str('') bool(false) false false true true true false bool(false) str('0') false false true true true false float(-INF) bool(true) false false true true true false bool(true) float(1) false false true true true false float(INF) bool(true) false false true true true false float(NAN) bool(true) false false true true true false bool(true) int(-1) false false true true true false bool(true) int(1) false false true true true false bool(true) str("\0") false false true true true false bool(true) str('+') false false true true true false bool(true) str('-') false false true true true false bool(true) str('01') false false true true true false bool(true) str('1') false false true true true false bool(true) str('false') false false true true true false str('text') bool(true) false false true true true false str('true') bool(true) false false true true true false int(0) float(0) false false true true true false str("\0") float(0) false false true true true false str('') float(0) false false true true true false str('+') float(0) false false true true true false str('-') float(0) false false true true true false str('0') float(0) false false true true true false str('false') float(0) false false true true true false str('text') float(0) false false true true true false str('true') float(0) false false true true true false int(1) float(1) false false true true true false float(1) str('01') false false true true true false float(1) str('1') false false true true true false str("\0") int(0) false false true true true false str('') int(0) false false true true true false str('+') int(0) false false true true true false str('-') int(0) false false true true true false int(0) str('0') false false true true true false str('false') int(0) false false true true true false str('text') int(0) false false true true true false str('true') int(0) false false true true true false int(1) str('01') false false true true true false int(1) str('1') false false true true true false str('1') str('01') false false true true true falseLower and Greater at the same time?
$a VS. $b $a>$b $a<$b $a<=$b $a>=$b $a==$b $a===$b float(NAN) str("\0") true true true true false false float(NAN) str('') true true true true false false float(NAN) str('+') true true true true false false float(NAN) str('-') true true true true false false float(NAN) str('0') true true true true false false float(NAN) str('01') true true true true false false float(NAN) str('1') true true true true false false float(NAN) str('false') true true true true false false float(NAN) str('text') true true true true false false float(NAN) str('true') true true true true false falseEqual AND identical:
$a VS. $b $a>$b $a<$b $a<=$b $a>=$b $a==$b $a===$b NULL(NULL) NULL(NULL) false false true true true true float(-INF) float(-INF) false false true true true true float(INF) float(INF) false false true true true trueLower or Greater:
$a VS. $b $a>$b $a<$b $a<=$b $a>=$b $a==$b $a===$bNULL(NULL) bool(true) false true true false false false float(-INF) NULL(NULL) true false false true false false NULL(NULL) float(1) false true true false false false float(INF) NULL(NULL) true false false true false false float(NAN) NULL(NULL) true false false true false false NULL(NULL) int(-1) false true true false false false NULL(NULL) int(1) false true true false false false NULL(NULL) str("\0") false true true false false false NULL(NULL) str('+') false true true false false false NULL(NULL) str('-') false true true false false false NULL(NULL) str('0') false true true false false false NULL(NULL) str('01') false true true false false false NULL(NULL) str('1') false true true false false false NULL(NULL) str('false') false true true false false false NULL(NULL) str('text') false true true false false false NULL(NULL) str('true') false true true false false false array() bool(true) false true true false false false float(-INF) array() false true true false false false array() float(0) true false false true false false array() float(1) true false false true false false float(INF) array() false true true false false false float(NAN) array() false true true false false false array() int(-1) true false false true false false array() int(0) true false false true false false array() int(1) true false false true false false array() str("\0") true false false true false false str('') array() false true true false false false array() str('+') true false false true false false array() str('-') true false false true false false array() str('0') true false false true false false array() str('01') true false false true false false array() str('1') true false false true false false array() str('false') true false false true false false array() str('text') true false false true false false array() str('true') true false false true false false bool(true) bool(false) true false false true false false float(-INF) bool(false) true false false true false false float(1) bool(false) true false false true false false float(INF) bool(false) true false false true false false float(NAN) bool(false) true false false true false false bool(false) int(-1) false true true false false false int(1) bool(false) true false false true false false bool(false) str("\0") false true true false false false bool(false) str('+') false true true false false false bool(false) str('-') false true true false false false bool(false) str('01') false true true false false false str('1') bool(false) true false false true false false bool(false) str('false') false true true false false false str('text') bool(false) true false false true false false str('true') bool(false) true false false true false false bool(true) float(0) true false false true false false bool(true) int(0) true false false true false false str('') bool(true) false true true false false false bool(true) str('0') true false false true false false float(-INF) float(0) false true true false false false float(-INF) float(1) false true true false false false float(INF) float(-INF) true false false true false false float(-INF) int(-1) false true true false false false float(-INF) int(0) false true true false false false float(-INF) int(1) false true true false false false float(-INF) str("\0") false true true false false false float(-INF) str('') false true true false false false float(-INF) str('+') false true true false false false float(-INF) str('-') false true true false false false float(-INF) str('0') false true true false false false float(-INF) str('01') false true true false false false float(-INF) str('1') false true true false false false float(-INF) str('false') false true true false false false float(-INF) str('text') false true true false false false float(-INF) str('true') false true true false false false float(1) float(0) true false false true false false float(INF) float(0) true false false true false false float(0) int(-1) true false false true false false int(1) float(0) true false false true false false float(0) str('01') false true true false false false str('1') float(0) true false false true false false float(INF) float(1) true false false true false false float(1) int(-1) true false false true false false float(1) int(0) true false false true false false float(1) str("\0") true false false true false false str('') float(1) false true true false false false float(1) str('+') true false false true false false float(1) str('-') true false false true false false float(1) str('0') true false false true false false float(1) str('false') true false false true false false str('text') float(1) false true true false false false str('true') float(1) false true true false false false float(INF) int(-1) true false false true false false float(INF) int(0) true false false true false false float(INF) int(1) true false false true false false float(INF) str("\0") true false false true false false float(INF) str('') true false false true false false float(INF) str('+') true false false true false false float(INF) str('-') true false false true false false float(INF) str('0') true false false true false false float(INF) str('01') true false false true false false float(INF) str('1') true false false true false false float(INF) str('false') true false false true false false float(INF) str('text') true false false true false false float(INF) str('true') true false false true false false int(0) int(-1) true false false true false false int(1) int(-1) true false false true false false str("\0") int(-1) true false false true false false str('') int(-1) true false false true false false str('+') int(-1) true false false true false false str('-') int(-1) true false false true false false str('0') int(-1) true false false true false false int(-1) str('01') false true true false false false str('1') int(-1) true false false true false false str('false') int(-1) true false false true false false str('text') int(-1) true false false true false false str('true') int(-1) true false false true false false int(1) int(0) true false false true false false int(0) str('01') false true true false false false str('1') int(0) true false false true false false int(1) str("\0") true false false true false false str('') int(1) false true true false false false int(1) str('+') true false false true false false int(1) str('-') true false false true false false int(1) str('0') true false false true false false int(1) str('false') true false false true false false str('text') int(1) false true true false false false str('true') int(1) false true true false false false str('') str("\0") false true true false false false str('+') str("\0") true false false true false false str('-') str("\0") true false false true false false str("\0") str('0') false true true false false false str("\0") str('01') false true true false false false str('1') str("\0") true false false true false false str('false') str("\0") true false false true false false str('text') str("\0") true false false true false false str('true') str("\0") true false false true false false str('') str('+') false true true false false false str('') str('-') false true true false false false str('') str('0') false true true false false false str('') str('01') false true true false false false str('') str('1') false true true false false false str('') str('false') false true true false false false str('') str('text') false true true false false false str('') str('true') false true true false false false str('-') str('+') true false false true false false str('+') str('0') false true true false false false str('+') str('01') false true true false false false str('1') str('+') true false false true false false str('false') str('+') true false false true false false str('text') str('+') true false false true false false str('true') str('+') true false false true false false str('-') str('0') false true true false false false str('-') str('01') false true true false false false str('1') str('-') true false false true false false str('false') str('-') true false false true false false str('text') str('-') true false false true false false str('true') str('-') true false false true false false str('0') str('01') false true true false false false str('1') str('0') true false false true false false str('false') str('0') true false false true false false str('text') str('0') true false false true false false str('true') str('0') true false false true false false str('false') str('01') true false false true false false str('text') str('01') true false false true false false str('true') str('01') true false false true false false str('1') str('false') false true true false false false str('text') str('1') true false false true false false str('true') str('1') true false false true false false str('text') str('false') true false false true false false str('true') str('false') true false false true false false str('true') str('text') true false false true false false
$a > $b > $cConundrum when:$ais not greater than$c.A<C : float(NAN) > str('a') > str('') A<C : float(NAN) > str('a') > str('1') A<C : float(NAN) > str('a') > str('A') A<C : float(NAN) > str('a') > str('0') A<C : float(NAN) > str('1') > str('') A<C : float(NAN) > str('1') > str('0') A<C : float(NAN) > str('A') > str('') A<C : float(NAN) > str('A') > str('1') A<C : float(NAN) > str('A') > str('0') A<C : float(NAN) > str('0') > str('') A<C : str('') > float(NAN) > str('a') A<C : str('') > float(NAN) > str('1') A<C : str('') > float(NAN) > str('A') A<C : str('') > float(NAN) > str('0') A<C : str('a') > str('') > float(NAN) A<C : str('a') > str('1') > float(NAN) A<C : str('a') > str('A') > float(NAN) A<C : str('a') > str('0') > float(NAN) A<C : str('0') > str('') > float(NAN) A==C : bool(true) > str('') > float(NAN) A==C : bool(true) > str('') > float(-INF) A==C : bool(true) > str('') > int(-1) A==C : bool(true) > str('') > float(-1) A==C : bool(true) > array() > float(NAN) A==C : bool(true) > array() > float(INF) A==C : bool(true) > array() > float(-INF) A==C : bool(true) > array() > str('a') A==C : bool(true) > array() > int(1) A==C : bool(true) > array() > float(1) A==C : bool(true) > array() > str('1') A==C : bool(true) > array() > str('A') A==C : bool(true) > array() > int(-1) A==C : bool(true) > array() > float(-1) A==C : bool(true) > int(0) > float(-INF) A==C : bool(true) > int(0) > int(-1) A==C : bool(true) > int(0) > float(-1) A==C : bool(true) > str('0') > float(NAN) A==C : bool(true) > str('0') > float(-INF) A==C : bool(true) > str('0') > int(-1) A==C : bool(true) > str('0') > float(-1) A==C : bool(true) > float(0) > float(-INF) A==C : bool(true) > float(0) > int(-1) A==C : bool(true) > float(0) > float(-1) A==C : int(1) > str('a') > str('1') A==C : int(1) > str('A') > str('1') A==C : float(1) > str('a') > str('1') A==C : float(1) > str('A') > str('1') A==C : str('a') > str('1') > int(0) A==C : str('a') > str('1') > float(0) A==C : str('') > float(-INF) > NULL(NULL) A==C : str('') > float(-INF) > bool(false) A==C : str('') > int(-1) > NULL(NULL) A==C : str('') > int(-1) > bool(false) A==C : str('') > float(-1) > NULL(NULL) A==C : str('') > float(-1) > bool(false) A==C : array() > float(NAN) > NULL(NULL) A==C : array() > float(NAN) > bool(false) A==C : array() > float(INF) > NULL(NULL) A==C : array() > float(INF) > bool(false) A==C : array() > float(-INF) > NULL(NULL) A==C : array() > float(-INF) > bool(false) A==C : array() > str('a') > NULL(NULL) A==C : array() > str('a') > bool(false) A==C : array() > int(1) > NULL(NULL) A==C : array() > int(1) > bool(false) A==C : array() > float(1) > NULL(NULL) A==C : array() > float(1) > bool(false) A==C : array() > str('1') > NULL(NULL) A==C : array() > str('1') > bool(false) A==C : array() > str('A') > NULL(NULL) A==C : array() > str('A') > bool(false) A==C : array() > str('0') > NULL(NULL) A==C : array() > int(-1) > NULL(NULL) A==C : array() > int(-1) > bool(false) A==C : array() > float(-1) > NULL(NULL) A==C : array() > float(-1) > bool(false) A==C : str('') > float(NAN) > bool(false) A==C : str('') > float(NAN) > NULL(NULL) A==C : str('A') > str('1') > int(0) A==C : str('A') > str('1') > float(0) A==C : int(0) > float(-INF) > NULL(NULL) A==C : int(0) > float(-INF) > bool(false) A==C : int(0) > int(-1) > NULL(NULL) A==C : int(0) > int(-1) > bool(false) A==C : int(0) > float(-1) > NULL(NULL) A==C : int(0) > float(-1) > bool(false) A==C : str('0') > float(NAN) > bool(false) A==C : str('0') > float(-INF) > bool(false) A==C : str('0') > int(-1) > bool(false) A==C : str('0') > float(-1) > bool(false) A==C : float(0) > float(-INF) > NULL(NULL) A==C : float(0) > float(-INF) > bool(false) A==C : float(0) > int(-1) > NULL(NULL) A==C : float(0) > int(-1) > bool(false) A==C : float(0) > float(-1) > NULL(NULL) A==C : float(0) > float(-1) > bool(false) A===C : str('0') > float(NAN) > str('0') A===C : str('') > float(NAN) > str('') A===C : str('a') > float(NAN) > str('a') A===C : str('1') > float(NAN) > str('1') A===C : str('A') > float(NAN) > str('A')Fun string comparison: 'Queen'
>'King'>'Jack'>'Ace'Also check out PHP type comparison tables covering pairs:
isset()andis_null()if()andempty()- boolean
==vs.===
Check the differences between PHP versions live at. http://3v4l.org/MAfDu.
讨论(0) - Check both variable types with
-
PHP's comparison operators deviate from the computer-scientific definitions in several ways:
In order to constitute an equivalence relation
==has to be reflexive, symmetric and transitive:PHP's
==operator is not reflexive, i.e.$a == $ais not always true:var_dump(NAN == NAN); // bool(false)Note: The fact that any comparison involving
NANis alwaysfalseis not specific to PHP. It is mandated by the IEEE 754 Standard for Floating-Point Arithmetic (more info).PHP's
==operator is symmetric, i.e.$a == $band$b == $aare always the same.PHP's
==operator is not transitive, i.e. from$a == $band$b == $cdoes not follows$a == $c:var_dump(true == "a"); // bool(true) var_dump("a" == 0); // bool(true) var_dump(true == 0); // bool(false)
In order to constitute a partial order
<=/>=has to be reflexive, anti-symmetric and transitive:PHP's
<=operator is not reflexive, i.e.$a <= $ais not always true (Example same as for==).PHP's
<=operator is not anti-symmetric, i.e. from$a <= $band$b <= $adoes not follow$a == $b:var_dump(NAN <= "foo"); // bool(true) var_dump("foo" <= NAN); // bool(true) var_dump(NAN == "foo"); // bool(false)PHP's
<=operator is not transitive, i.e. from$a <= $band$b <= $cdoes not follow$a <= $c(Example same as for==).Extra: PHP's
<=operator is not total, i.e. both$a <= $band$b <= $acan be false:var_dump(new stdClass <= new DateTime); // bool(false) var_dump(new DateTime <= new stdClass); // bool(false)
In order to constitute a strict partial order
</>has to be irreflexive, asymmetric and transitive:PHP's
<operator is irreflexive, i.e.$a < $ais never true. Note that this is true only as of PHP 5.4. PreviouslyINF < INFevaluated totrue.PHP's
<operator is not asymmetric, i.e. from$a < $bdoes not follow!($b < $a)(Example same as for<=not being anti-symmetric).PHP's
<operator is not transitive, i.e. from$a < $band$b < $cdoes not follow$a < $c:var_dump(-INF < 0); // bool(true) var_dump(0 < TRUE); // bool(true) var_dump(-INF < TRUE); // bool(false)Extra: PHP's
<operator is not trichotomous, i.e. all of$a < $b,$b < $aand$a == $bcan be false (Example same as for<=not being total).Extra: PHP's
<operator can be circular, i.e. it is possible that$a < $b,$b < $cand$c < $a:var_dump(INF < []); // bool(true) var_dump([] < new stdClass); // bool(true) var_dump(new stdClass < INF); // bool(true)Note: The above example throws a "Object of class stdClass could not be converted to double" notice.
You can find a few nice graphs for PHP's comparison operators on PHP Sadness 52 - Comparison operators.
As a last note, I want to point out that there are two equalities that PHP does guarantee (unlike pretty much everything else). These two always hold, simply because the compiler reduces one to the other:
($a > $b) == ($b < $a) ($a >= $b) == ($b <= $a)讨论(0) -
After your correction of the second part of your question, I leave the answer to that part to the others. I just want to give the most surprising answer to the first part of your question, i.e., whether there is an example of the
<and>operators being intransitive. Here it is.These are all
true:"10" < "1a" "1a" < "2" "10" > "2"If
<were transitive ($a < $b∧$b < $c⇒$a < $c), the last line would be"10" < "2"but PHP tries to be kind (?!) and interpret strings as numbers whenever it can.
It turns out that, because of the above intransitivity,
sort()can sort the same elements into a different order depending on their input order, even when no two elements are==(and no element is NAN). I pointed this out in a comment to sort(), the essence of which is:sort(array("10", "1a", "2" )) => array("10", "1a", "2" ) sort(array("10", "2", "1a")) => array("1a", "2", "10") sort(array("1a", "10", "2" )) => array("2", "10", "1a") sort(array("1a", "2", "10")) => array("1a", "2", "10") sort(array("2", "10", "1a")) => array("2", "10", "1a") sort(array("2", "1a", "10")) => array("10", "1a", "2" )讨论(0)
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