added email validation.

.idea/php.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="PhpProjectSharedConfiguration" php_language_level="7" />
+</project>

.idea/vcs.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>

Validate.php

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+<?php
+
+/**
+ * User: Marcel 'CoNfu5eD Naeve <confu5ed@serious-pro.de>
+ * Date: 03.05.2016
+ * Time: 21:28
+ */
+
+/**
+ * Class Validate.
+ */
+class Validate
+{
+    /**
+     * Static function for validating email addresses with multiple secure levels and blacklist.
+     * @param string $email email address
+     * @param int $level secure level
+     * @param array $lists filter lists like whitelist, blacklist, dnsbl,..
+     * @return bool is the email address valid?
+     */
+    static function email (string $email, int $level=1, array $lists=[]) : bool
+    {
+        $results = true;
+        $email = trim($email); // sanitize before validation
+        $split = explode('@', $email);
+
+        // Whitelisted?
+        if(isset($lists['whitelist'])) {
+            if(in_array($email, $lists['whitelist']))
+                return true;
+        }
+
+        // Blacklisted?
+        if(isset($lists['blacklist'])) {
+            if(in_array($email, $lists['blacklist']))
+                return false;
+        }
+
+        // check format
+        if($results && $level >= 1) {
+            if(filter_var($email, FILTER_VALIDATE_EMAIL) === false)
+            {
+                $results = false;
+            } else {
+                $results = true;
+            }
+        }
+
+        // check for dns MX entry
+        if($results && $level >= 2) {
+            $results = checkdnsrr($split[1], "MX");
+        }
+
+        // DNS Blacklist (domains)
+        if($results && isset($lists['dnsbl']) && $level >= 3) {
+            // TODO: check if domain is registered in dnsbl of list.
+        }
+
+        return $results;
+    }
+}