//check that the string starts with a digit, and contains only alphanumeric characters.
$pattern = '/^[0-9][0-9A-Za-z]*/';
$str1 = '5fun4all';
$str2 = 'fun4all5';
if (preg_match($pattern, $subject)) {
    //more about preg_functions coming up
}

//The pattern matches '5fun4all' but not 'fun4all5'