How to validate email with PHP

validate-email.php

<?php
// inspired by a note at http://php.net/manual/en/function.getmxrr.php
// further inspiration from https://github.com/webdigi/SMTP-Based-Email-Validation
function validateEmail($email)
{
  $emailValid = false;

  $domain = extractFullyQualifiedDomainFromEmail($email);
  $mxHost = findPreferredMxHostForDomain($domain);

  if ($mxHost) {
//    echo $mxHost . "<br>";
    $mxSocket = @fsockopen($mxHost, 25, $errno, $errstr, 2);
    if ($mxSocket) {
      $response = "";
      // say HELO to mailserver
      $response .= sendCommand($mxSocket, "EHLO mx1.validemail.com");
      // initialize sending mail
      $response .= sendCommand($mxSocket, "MAIL FROM:<info@validemail.com>");
      // try recipient address, will return 250 when ok..
      $rcptText = sendCommand($mxSocket, "RCPT TO:<" . $email . ">");
      $response .= $rcptText;
      if (substr($rcptText, 0, 3) == "250") {
        $emailValid = true;
      }
      // quit mail server connection
      sendCommand($mxSocket, "QUIT");
      fclose($mxSocket);
    }
  }
  return $emailValid;
}

function extractFullyQualifiedDomainFromEmail($email)
{
  $mailSegments = explode("@", $email);
  $domain = $mailSegments[1];
  // http://stackoverflow.com/q/14065946/131929
  // fully qualified domain names should end with a '.', DNS resolution may otherwise take a very long time
  if (substr($domain, -1) != ".") {
    return $domain . ".";
  }
  return $domain;
}

function findPreferredMxHostForDomain($domain)
{
  $mxRecordsAvailable = getmxrr($domain, $mxRecords, $mxWeight);
  if ($mxRecordsAvailable) {
    // copy mx records and weight into array $mxHosts
    $mxHosts = array();
    for ($i = 0; $i < count($mxRecords); $i++) {
      $mxHosts[$mxRecords[$i]] = $mxWeight[$i];
    }
    asort($mxHosts, SORT_NUMERIC);
    reset($mxHosts);

    return array_keys($mxHosts)[0];
  } else {
    return null;
  }
}

function sendCommand($mxSocket, $command)
{
//  print htmlspecialchars($command) . "<br>";
  fwrite($mxSocket, $command . "\r\n");

  $response = "";
  stream_set_timeout($mxSocket, 1);

  // Wait at most 10 * timeout for the server to respond.
  // In most cases the response arrives within timeout time and, therefore, there's no need to wait any longer in such
  // cases (i.e. keep timeout short). However, there are cases when a server is slow to respond and that for-loop will
  // give it some slack. See http://stackoverflow.com/q/36961265/131929 for the whole story.
  for ($i = 0; $i < 10; $i++) {
    while ($line = fgets($mxSocket)) {
      $response .= $line;
    }
    // Only continue the for-loop if the server hasn't sent anything yet.
    if ($response != "") {
      break;
    }
  }

//  print nl2br($response);
  return $response;
}

?>