The Zend_Validate component provides a set of commonly needed validators. It also provides a simple validator chaining mechanism by which multiple validators may be applied to a single datum in a user-defined order.

What is a validator?

A validator examines its input with respect to some requirements and produces a boolean result - whether the input successfully validates against the requirements. If the input does not meet the requirements, a validator may additionally provide information about which requirement(s) the input does not meet.

For example, a web application might require that a username be between six and twelve characters in length and may only contain alphanumeric characters. A validator can be used for ensuring that usernames meet these requirements. If a chosen username does not meet one or both of the requirements, it would be useful to know which of the requirements the username fails to meet.

Having defined validation in this way provides the foundation for Zend_Validate_Interface, which defines two methods, isValid() and getMessages(). The isValid() method performs validation upon the provided value, returning true if and only if the value passes against the validation criteria. If isValid() returns false, the getMessages() is provided to return an array of messages explaining the reason(s) for validation failure.

The following example illustrates validation of an e-mail address:

<?php

require_once 'Zend/Validate/EmailAddress.php';
$validator = new Zend_Validate_EmailAddress();
if ($validator->isValid($email)) {
    // email appears to be valid
} else {
    // email is invalid; print the reasons
    foreach ($validator->getMessages() as $message) {
        echo "$message\n";
    }
}

?>