Validator Class

index.php

<?php

$validator = new Validator();

// Each validation rule is a property of the validator object.
$validator->username = function($value, $key, $self)
{
	if(preg_match('~\W~', $value))
	{
		return 'Your username must only contain letters and numbers';
	}
}

$validator->password = function($value, $key, $self)
{
	if(strlen($value) < 8)
	{
		return 'Your password must be at least 8 characters long';
	}
};


// $_POST = array('username' => ..., 'password' => ...)
if( ! $validator($_POST))
{
	// Return a JSON object with the validation errors (great for jQuery)
	die(json_encode($validator->errors()));
}
else
{
	// Save new user to database
}

validator.php

<?php
/**
 * Validator class based on anonymous functions.
 *
 * @see http://php.net/manual/en/functions.anonymous.php
 */
class Validator
{
	public $errors;

	/**
	 * Validate the given array of data using the functions set
	 *
	 * @param array $data to validate
	 * @return array
	 */
	public function __invoke(array $data)
	{
		unset($this->errors);

		$errors = array();
		foreach((array) $this as $key => $function)
		{
			$value = NULL;

			if(isset($data[$key]))
			{
				$value = $data[$key];
			}

			$error = $function($value, $key, $this);

			if($error)
			{
				$errors[$key] = $error;
			}
		}

		$this->errors = $errors;
		return ! $errors;
	}


	/**
	 * Return the validator errors
	 *
	 * @return array
	 */
	public function errors()
	{
		return $this->errors;
	}
}