PHP Version
<?php
  function birthday($user) {
    if(strftime('%m %d')==strftime('%m %d',$user['bday'])) 
      $age = strftime('%Y') - strftime('%Y',$user['bday']);
      if(($age%100)>10 && ($age%100)<20) $ap='th';
      else switch($age%10) {
        case 1: $ap = 'st'; break;
        case 2: $ap = 'nd'; break;
        case 3: $ap = 'rd'; break;
        default:  $ap = 'th'; break; 
      }
      return $age.$ap;
    else 
      return false;
  }
?>
Other than a little bit of added parameter parsing, if you compare the C version of birthday() to the PHP version, it isn't that much harder to write it in C.

C Version
PHP_FUNCTION(birthday) {
  time_t timestamp, now;
  struct tm *ta1, tmbuf1, *ta2, tmbuf2;
  int age;
  char ret_age[8];

  if (zend_parse_parameters(1 TSRMLS_CC, "l", &timestamp) == FAILURE)
      return;

  ta1 = php_localtime_r(&timestamp, &tmbuf1);
  time(&now);
  ta2 = php_localtime_r(&now, &tmbuf2);

  if(tmbuf1.tm_mday==tmbuf2.tm_mday && tmbuf1.tm_mon==tmbuf2.tm_mon) {
      age = tmbuf2.tm_year - tmbuf1.tm_year;
      if((age%100)>10 && (age%100)<19) sprintf(ret_age,"%dth",age);
      else switch(age % 10) {
          case 1: sprintf(ret_age,"%dst",age); break;
          case 2: sprintf(ret_age,"%dnd",age); break;
          case 3: sprintf(ret_age,"%drd",age); break;
          default:sprintf(ret_age,"%dth",age); break;
      }
  } else {
      RETURN_FALSE;
  }
  RETURN_STRING(ret_age,1);
}