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phpevaluationoperator-precedence

Order of evaluation of function arguments in PHP

Is order of evaluation of PHP function arguments guaranteed to be always the same?

Thanks.

Solution

  • Usually, yes. As the manual states:

    [Function] arguments are evaluated from left to right.

    But there are two edge cases where arguments are not evaluated at all:

    Undefined functions

    $calls = 0;
register_shutdown_function(function () use (&$calls) {
    echo $calls;
});
func_does_not_exist($calls++);

    This outputs 0 on all versions of PHP.

    Missing constructor, a special case of an undefined function

    class Foo {}

$bar = 0;
$foo = new Foo($bar++);
echo $bar;

    This outputs 0 on PHP < 7.1, and 1 on PHP >= 7.1. It's been called the "Rasmus optimization", and it occurs only in the case of constructing classes without formal constructors. See also #67829, #54162 and #54170.

    In summary, the manual is correct. For defined functions, arguments are evaluated left-to-right then passed into the function. Undefined functions, for which a non-existent constructor is a special case, do not qualify as functions and so the evaluation before calling is itself undefined.