What is wrong with the PHP function in_array(...)? [duplicate]

Question

The PHP function in_array(...) "checks if a value exists in an array".

But I'm observing a very strange behavior on handling strings (PHP v7.0.3). This code

$needle = 'a';
$haystacks = [['a'], ['b'], [123], [0]];
foreach ($haystacks as $haystack) {
    $needleIsInHaystack = in_array($needle, $haystack);
    var_dump($needleIsInHaystack);
}

generates following output:

bool(true)
bool(false)
bool(false)
bool(true) <- WHAT?

The function returns true for every string $needle, if the $haystack contains an element with the value 0!

Is it really by design? Or is it a bug and should be reported?

Answer 1

If you do not set the third parameter of in_array to true, comparison is done using type coercion.

If the third parameter strict is set to TRUE then the in_array() function will also check the types of the needle in the haystack.

Under loose comparison rules, effectively 'a' is equal to 0 since (int)'a' == 0.