How to intentionally cause a "Fatal error: Allowed memory size of xxx bytes exhausted"

Should eat all memory.

$a = 'x';
while (true) {
    $a = $a.$a;
}

Here's the problem:

$test = "a" + $test;

+ in PHP is for arithmetic, not string concatination. Use:

$test = "a" . $test;

str_pad("",PHP_INT_MAX);