问题
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php == vs === operator
An easy answer for someone I'm sure. Can someone explain why this expression evaluates to true?
(1234 == '1234 test')
回答1:
Because you are using the == (similarity) operator and PHP is coercing the string to an int.
To resolve it use the === (equality) operator, which checks not only if the value is the same, but also if the data type is the same, so "123" string and 123 int won't be considered equal.
回答2:
In PHP (and JavaScript -- which has slightly different behavior), the comparison operator == works differently than it does in strongly-typed languages like C or Java. The === operator has the behavior that you most likely expect. Below is a breakdown of the two comparison operators as they apply to PHP.
==
This operator is officially known as the "equality" operator, though that doesn't really fit the normal definition of the word "equality". It does what is known as a type-juggling comparison. If the types of both operands don't match (in your example, 1234 was an integer and 1234 test was a string), PHP will implicitly cast the operands to each others' types and test the equality of the newly-typed values as shown below:
<?php
var_dump( (int) 'hi' ); // int(0)
var_dump( (string) 0 ); //string("0")
var_dump( 'hi' == 0 ); // bool(true)
var_dump( (int) '1hi' ); // int(1)
var_dump( 1 == '1hi' ); // bool(true)
It has a counterpart (type-juggling) inequality operator, !=.
===
The === operator, known as the "identical" operator, performs a strict check of the value and type of both operands and does not perform any implicit casts. Therefore, "0" does not === 0 and "1234 test"does not === 1234.
<?php
var_dump( '1234 test' === 1234 ); // bool(false)
It has a counterpart (strict) inequality operator, !==.
Quirks
Note that the === operator has behavior on objects that is considered strange by some. Say we have class A and variables $a and $b as defined below:
<?php
class A {
public $property = 'default value';
}
$a = new A();
$b = new A();
You might expect var_dump($a === $b); to output bool(true). It will actually return false. When used upon objects, the operator actually checks if both operands are references to the same object. The == operator, in this instance, works by checking the properties of the objects, so $a == $b.
PHP Manual Links
- Comparison operators
- Type juggling
回答3:
When casting a string to an integer, any numeric characters up to the first non-numeric character becomes the number. Thus '1234 test' becomes 1234 because space is not a numeric character.
Thus 1234 == '1234 test'
If you want to force a string comparison, you should cast to string:
''.(1234) == '1234 test' // implicit
(string) 1234 == '1234 test' // explicit
strval(1234) == '1234 test' // procedural
回答4:
You are loosely comparing two different types of data (an integer and a string). PHP has a very detailed chart of how comparisons work in their system when using the loose comparison binary operator (==):
http://php.net/manual/en/types.comparisons.php
If you want to ensure that the types are also in sync, that is that they are both integers or both strings, use the strong type comparison operator (===).
Note that, when using this operator, this will also return false:
1234 === '1234'
If you are unsure of your types when comparing, you can couple the strong-type comparison with PHP typecasting:
$a = 1234;
$b = '1234';
if ($a === $b) { } // Will not fire, as it is false
if ((int)$a === (int)$b) { } // Will fire, as it is true
回答5:
The double equals will tell php to parse an int from the string. The string will evaluate to the integer 1234. Use triple equals '===' to get exact comparison.
回答6:
If you compare a number with a string or the comparison involves numerical strings, then each string is converted to a number and the comparison performed numerically
var_dump(0 == "a"); // 0 == 0 -> true
来源：https://stackoverflow.com/questions/12151997/why-does-1234-1234-test-evaluate-to-true