gistfile1.php

<?php

function f($a,$k){
	if(!is_array($a) || !array_key_exists($k, $a)) return false;
	$s=$a[$k];
	$b=$a;
	$b[$k]=!$s;
	$t=$a[$k]===!$s;
	$a[$k]=$s;
	return $t;
}

$keys = array('string', 152, 0, '');
$values = array('string', 152, 0, '', true, false, !null, null, (object)[], array(), array('k' => 'v'));

// testing non-array «a»
foreach ($keys as $key) {
	foreach ($values as $value) {
		var_dump(f($value, $key) === false);
	}
}

// testing no-reference «a»
foreach ($keys as $key) {
	foreach ($values as $value) {
		var_dump(f(array($key => $value), $key) === false);
	}
}

// testing referencing «a»
foreach ($keys as $key) {
	foreach ($values as $value) {
		$array = array();
		$array[$key] =& $value;
		var_dump(f($array, $key) === true);
	}
}