Find string true or false

Is there a PHP function that returns either true or false if a string is within another string? All the functions I’ve found return either the part of the string that was found or false.

Well, use those functions,
and check if it ===false, if not the string is found and true

$pos = strrpos($mystring, "b");
if ($pos === false) { // note: three equal signs
    echo "NOT FOUND";
}else{
    echo "FOUND";
}

from http://php.net/manual/en/function.strrpos.php, you should realy read the code examples and maybe the comments too.