How to check, if a php string contains only english letters and digits?

if(ctype_alnum($string)) {
    echo "String contains only letters and numbers.";
}
else {
    echo "String doesn't contain only letters and numbers.";
}

Use preg_match().

if (!preg_match('/[^A-Za-z0-9]/', $string)) // '/[^a-z\d]/i' should also work.
{
  // string contains only english letters & digits
}