PHP if in_array() how to get the key as well?

array_search() is what you are looking for.

if (false !== $key = array_search(5, $array)) {
    // found!
} else {
    // not found!
}

If you only need the key of the first match, use array_search():

$key = array_search(5, $array);
if ($key !== false) {
    // Found...
}

If you need the keys of all entries that match a specific value, use array_keys():

$keys = array_keys($array, 5);
if (count($keys) > 0) {
    // At least one match...
}

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