Data validation is one of the most important and essential part of any data form. There are many ways to verify data input. Even for email addresses there could be several ways to verify it. In php we can use regex to verify email address. In the following code I've searched for an email address through regex email patteren in the provided value. If the value is found then we have an valid email address.

[Updated] The previous code wasn't much compatible with valid email addresses, I've updated the code, this code verifies email addresses wich are completely valid and according to the standards. (reference)

    function validEmail($email)
    {
       $isValid = true;
       $atIndex = strrpos($email, "@");
       if (is_bool($atIndex) && !$atIndex)
       {
          $isValid = false;
       }
       else
       {
          $domain = substr($email, $atIndex+1);
          $local = substr($email, 0, $atIndex);
          $localLen = strlen($local);
          $domainLen = strlen($domain);
          if ($localLen < 1 || $localLen > 64)
          {
             // local part length exceeded
             $isValid = false;
          }
          else if ($domainLen < 1 || $domainLen > 255)
          {
             // domain part length exceeded
             $isValid = false;
          }
          else if ($local[0] == '.' || $local[$localLen-1] == '.')
          {
             // local part starts or ends with '.'
             $isValid = false;
          }
          else if (preg_match('/\\.\\./', $local))
          {
             // local part has two consecutive dots
             $isValid = false;
          }
          else if (!preg_match('/^[A-Za-z0-9\\-\\.]+$/', $domain))
          {
             // character not valid in domain part
             $isValid = false;
          }
          else if (preg_match('/\\.\\./', $domain))
          {
             // domain part has two consecutive dots
             $isValid = false;
          }
          else if(!preg_match('/^(\\\\.|[A-Za-z0-9!#%&`_=\\/$\'*+?^{}|~.-])+$/', str_replace("\\\\","",$local)))
          {
             // character not valid in local part unless 
             // local part is quoted
             if (!preg_match('/^"(\\\\"|[^"])+"$/', str_replace("\\\\","",$local)))
             {
                $isValid = false;
             }
          }
       }
       return $isValid;
    }

Click here to test for your self.