Very illogical php value comparisons

Question

I stumbled upon a very strange bit of PHP code. Could someone explain why this is happening? *****BONUS POINTS***** if you can tell my why this is useful.

Answer 1

This is a basic principle of weakly/dynamically typed languages called type juggling. Types will be cast to other types in certain circumstances. When you compare a string to a number, the string will be cast into a number. When comparing anything to a boolean, that value will be cast to a boolean.

There are rules for every type as to how it will be cast into another type or how it compares to other types. 'a' happens to be converted to 0 when cast to a number (the only logical choice, really). To avoid this type casting, test not with the equality operator ==, but with the identity operator ===.

As James pointed out, this is useful since PHP deals a lot with strings that are really numbers. For example, HTML forms only submit strings, even if the value is a number. It also allows for some really terse code, like:

$result = someOperation();
if (!$result) {
    // $result may be null, false, 0, '' or array(),
    // all of which we're not interested in
    error();
}

It also means you have to be really careful about what to check for in which circumstances though, since a value might unexpectedly cast into something else. And admittedly, 'a' == 0 in itself is really a pitfall of type juggling rather than helpful. It's one of the situations where you have to be careful and test like if (is_numeric($var) && $var == 0).