What alternative PHP function could be used instead of strpos to improve the efficiency of the code?

Using the `stripos` function instead of `strpos` can improve the efficiency of the code as it performs a case-insensitive search for a substring within a string. This eliminates the need to convert both the string and substring to lowercase before using `strpos`.

$haystack = "Hello World";
$needle = "WORLD";

if (stripos($haystack, $needle) !== false) {
    echo "Substring found!";
} else {
    echo "Substring not found.";
}