Redirect if Email Exists

<?php
/**
 * Gravity Wiz // Gravity Forms // Redirect if Email Exists
 * https://gravitywiz.com/
 *
 * Redirect to a specified URL if the the submitted email address matches an existing user.
 *
 * Note: Does not work with AJAX-enabled forms.
 *
 * Plugin Name: Gravity Forms - Redirect if Email Exists
 * Plugin URI: https://gravitywiz.com/
 * Description: Redirect to a specified URL if the the submitted email address matches an existing user.
 * Author: Gravity Wiz
 * Version: 0.1
 * Author URI: https://gravitywiz.com
 */
class GW_Redirect_Email_Exists {

	private $_args = array();

	public function __construct( $args = array() ) {

		// set our default arguments, parse against the provided arguments, and store for use throughout the class
		$this->_args = wp_parse_args( $args, array(
			'form_id'        => false,
			'email_field_id' => false,
			'redirect_url'   => false,
		) );

		// do version check in the init to make sure if GF is going to be loaded, it is already loaded
		add_action( 'init', array( $this, 'init' ) );

	}

	public function init() {

		// make sure we're running the required minimum version of Gravity Forms
		if ( ! property_exists( 'GFCommon', 'version' ) || ! version_compare( GFCommon::$version, '1.8', '>=' ) ) {
			return;
		}

		add_filter( 'gform_validation', array( $this, 'maybe_redirect' ) );

	}

	public function maybe_redirect( $result ) {
		if ( $this->is_applicable_form( $result['form'] ) ) {
			$field = GFFormsModel::get_field( $result['form'], $this->_args['email_field_id'] );
			$value = rgpost( 'input_' . str_replace( '.', '_', $field->id ) );
			if ( get_user_by( 'email', $value ) ) {
				wp_redirect( $this->_args['redirect_url'] );
				exit;
			}
		}
		return $result;
	}

	public function is_applicable_form( $form ) {

		$form_id = isset( $form['id'] ) ? $form['id'] : $form;

		return empty( $this->_args['form_id'] ) || $form_id == $this->_args['form_id'];
	}

}

# Configuration

new GW_Redirect_Email_Exists( array(
	'form_id'        => 123,
	'email_field_id' => 4,
	'redirect_url'   => 'http://google.com',
) );