title: Codeigniter 3.1.7 valid_email issue
body: <p>I have two sites using <code>Codeigniter</code>. Today I have updated sites to <code>Codeigniter 3.1.7</code> (from <code>3.1.6</code>). After that at one of this sites email validation had broken, <code>valid_email</code> validation rule doesn't work any more. On second site all fine. I had check out error logs and found error: </p>
  
  <pre><code>ERROR - 2018-02-06 10:13:21 --&gt; Severity: Warning --&gt; idn_to_ascii() expects parameter 3 to be long, string given /public_html/system/libraries/Form_validation.php 1235
  </code></pre>
  
  <p>Codeigniter's changlog says:</p>
  
  <pre><code>Updated Form Validation Library rule valid_email to use INTL_IDNA_VARIANT_UTS46 for non-ASCII domain names.
  </code></pre>
  
  <p>It seems to me that <code>INTL_IDNA_VARIANT_UTS46</code> constant is not defined. Site using <code>PHP Version 5.6.30</code></p>
  
  <p>How can I fix this issue?</p>