[PHP]: What does array_search() return if nothing was found?

if you're just checking if the value exists, in_array is the way to go.

Quoting the manual page of array_search() :

Returns the key for needle if it is found in the array, FALSE otherwise.

Which means you have to use something like :

$found = array_search($needle, $haystack);

if ($found !== false) {
    // do stuff
    // when found
} else {
    // do different stuff
    // when not found
}

Note I used the !== operator, that does a type-sensitive comparison ; see Comparison Operators, Type Juggling, and Converting to boolean for more details about that ;-)