Order of evaluation of function arguments in PHP

Question

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

Thanks.

Answer 1

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.

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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.

Answer 2

From the manual:

Information may be passed to functions via the argument list, which is a comma-delimited list of expressions. The arguments are evaluated from left to right.

In theory it might change in future versions of PHP, but I certainly wouldn't expect it to.

(And please don't write any code that relies on it, for everyone's sake...)