gistfile1.php

<?php
function get_certificate_ip($ssl_key) {
  $ip_addresses = array('127.0.0.1', '192.168.0.2');

  $path = "/Users/adrian/test";
    $pattern = "{$path}/{$ssl_key}__*.assign";
    $files = glob($pattern);
    if (sizeof($files) == 1) {
      $pattern = "/^{$ssl_key}__(.*)\.assign$/";
      preg_match($pattern, basename($files[0]), $matches);
      return $matches[1];
    }
    elseif (sizeof($files) == 0) {
      foreach ($ip_addresses as $ip) {
        $pattern = "{$path}/*__{$ip}.assign";
        $files = glob($pattern);

        if (sizeof($files) == 0) {
          touch("{$path}/{$ssl_key}__{$ip}.assign");
          return $ip;
        }
      }
    }
    return FALSE; // generate error
}

print_r(get_certificate_ip("mykey1"));
print_r(get_certificate_ip("mykey2"));
print_r(get_certificate_ip("mykey3"));