$string = "233718_This_is_a_string";
$withCharacter = strstr($string, '_'); // "_This_is_a_string"
echo substr($withCharacter, 1); // "This_is_a_string"
In a single statement it would be.
echo substr(strstr("233718_This_is_a_string", '_'), 1); // "This_is_a_string"