[WordPress] Validating Input via the WordPress Settings API

1-interface-input-validator.php

<?php

interface Input_Validator {

	public function __construct( $setting );
	public function is_valid( $input );

}

2-class-address-validator.php

<?php

/**
 * This class is responsible for validating the address field
 * as provided by the user in the Dashboard.
 *
 * @since         1.0.0
 *
 * @implements    Input_Validator
 * @package       Acme/classes
 */
class Address_Validator implements Input_Validator {
	
	/**
	 * Slug title of the setting to which this error applies
	 * as defined via the implementation of the Settings API.
	 * 
	 * @access private
	 */
	private $setting;
	
	/**
	 * Creates an instance of the class and associates the
	 * specified setting with the property of this class.
	 * 
	 * @param    string    $setting    The title of the setting we're validating
	 */
	public function __construct( $setting ) {
		$this->setting = $setting;
	}
	
	/**
	 * Determines if the specified input is valid. For purposes of addresses,
	 * we only want to make sure the user isn't specifying an empty string.
	 * 
	 * @param    string    $input    The address string
	 * @return   bool                True if the input is valid; otherwise, false
	 */
	public function is_valid( $input ) {
		
		$is_valid = true;
		
		// If the input is an empty string, add the error message and mark the validity as false
		if ( '' == trim( $input ) ) {
			
			$this->add_error( 'invalid-address', 'You must provide a valid address.' );
			$is_valid = false;
		}
		
		return $is_valid;
		
	}
	
	/**
	 * Adds an error message to WordPress' error collection to be displayed in the dashboard.
	 * 
	 * @access   private
	 * 
	 * @param    string    $key        The key to which the specified message will be associated
	 * @param    string    $message    The message to display in the dashboard
	 */
	private function add_error( $key, $message ) {
		
		add_settings_error(
			$this->setting,
			$key,
			$message,
			'error'
		);
		
	}
	
}

3-class-city-validator.php

<?php

/**
 * This class is responsible for validating the city field
 * as provided by the user in the Dashboard.
 *
 * @since         1.0.0
 *
 * @implements    Input_Validator
 * @package       Acme/classes
 */
class City_Validator implements Input_Validator {
	
	/**
	 * Slug title of the setting to which this error applies
	 * as defined via the implementation of the Settings API.
	 * 
	 * @access private
	 */
	private $setting;
	
	/**
	 * Creates an instance of the class and associates the
	 * specified setting with the property of this class.
	 * 
	 * @param    string    $setting    The title of the setting we're validating
	 */
	public function __construct( $setting ) {
		$this->setting = $setting;
	}
	
	/**
	 * Determines if the specified input is valid. For purposes of cities,
	 * we only want to make sure the user isn't specifying an empty string.
	 * This can be more complicated, though, if you wanted to compare it against
	 * an array of cities, etc.
	 * 
	 * @param    string    $input    The city string
	 * @return   bool                True if the input is valid; otherwise, false
	 */
	public function is_valid( $input ) {
		
		$is_valid = true;
		
		// If the input is an empty string, add the error message and mark the validity as false
		if ( '' == trim( $input ) ) {
			
			$this->add_error( 'invalid-city', 'You must provide a valid city.' );
			$is_valid = false;
		}
		
		return $is_valid;
		
	}
	
	/**
	 * Adds an error message to WordPress' error collection to be displayed in the dashboard.
	 * 
	 * @access   private
	 * 
	 * @param    string    $key        The key to which the specified message will be associated
	 * @param    string    $message    The message to display in the dashboard
	 */
	private function add_error( $key, $message ) {
		
		add_settings_error(
			$this->setting,
			$key,
			$message,
			'error'
		);
		
	}
	
}

4-postal-code-validator.php

<?php

/**
 * This class is responsible for validating a postal code
 * as provided by the user in the Dashboard.
 *
 * @since         1.0.0
 *
 * @implements    Input_Validator
 * @package       Acme/classes
 */
class Postal_Code_Validator implements Input_Validator {
	
	/**
	 * Slug title of the setting to which this error applies
	 * as defined via the implementation of the Settings API.
	 * 
	 * @access private
	 */
	private $setting;
	
	/**
	 * Creates an instance of the class and associates the
	 * specified setting with the property of this class.
	 * 
	 * @param    string    $setting    The title of the setting we're validating
	 */
	public function __construct( $setting ) {
		$this->setting = $setting;
	}
	
	/**
	 * Determines if the specified input is valid. For these postal codes,
	 * we're verifying the input that the user provides against a regular
	 * expression that verifies it's a valid Canadian postal code
	 * 
	 * @param    string    $input    The postal code string
	 * @return   bool                True if the input is valid; otherwise, false
	 */
	public function is_valid( $input ) {
		
		$is_valid = true;
		
		// Use the following RegEx to determine if the specified zip code is of proper Canadian format
		$is_valid =
			preg_match (
				'$[ABCEGHJKLMNPRSTVXY][0-9][ABCEGHJKLMNPRSTVWXYZ] ?[0-9][ABCEGHJKLMNPRSTVWXYZ][0-9]$',
				$input
			);
		
		// If the input is an empty string, add the error message and mark the validity as false
		if ( ! $is_valid ) {
			
			$this->add_error( 'invalid-postal-code', 'You must provide a valid postal code.' );
			$is_valid = false;
		}
		
		return $is_valid;
		
	}
	
	/**
	 * Adds an error message to WordPress' error collection to be displayed in the dashboard.
	 * 
	 * @access   private
	 * 
	 * @param    string    $key        The key to which the specified message will be associated
	 * @param    string    $message    The message to display in the dashboard
	 */
	private function add_error( $key, $message ) {
		
		add_settings_error(
			$this->setting,
			$key,
			$message,
			'error'
		);
		
	}
	
}