advanced_facebook_email_on_checkout.php

<?php

//Add from here down to the bottom of your PMProcustomizations plugin

function my_pmprorh_init() {
	// don't break if Register Helper is not loaded.
	if ( ! function_exists( 'pmprorh_add_registration_field' ) ) {
		return false;
	}

	// define the fields.
	$facebook_email_different = new PMProRH_Field(
		'facebook_email_different',              // input name, will also be used as meta key.
		'checkbox',                        // type of field.
		array(
			'label'    => 'Different Facebook Email    ',  // custom field label.
			'text'     => '',
			'hint'     => 'Check if the email associated with your Facebook account is different than the email address above',
		)
	);

	$facebook_email = new PMProRH_Field(
		'facebook_email',              // input name, will also be used as meta key.
		'text',                        // type of field.
		array(
			'label'    => 'Email Associated with Facebook Account',  // custom field label.
			'size'     => 40,            // input size.
			'profile'  => true,          // show in user profile.
			'depends'  => array(
				array(
					'id'    => 'facebook_email_different',
					'value' => true,
				),
			),
		)
	);

	pmprorh_add_registration_field(
		'after_email',  // location on checkout page.
		$facebook_email_different    // PMProRH_Field object.
	);

	pmprorh_add_registration_field(
		'after_email',  // location on checkout page.
		$facebook_email    // PMProRH_Field object.
	);

}
add_action( 'init', 'my_pmprorh_init' );

function my_pmprorh_validate( $pmpro_continue_registration ) {
	if ( ! isset( $_REQUEST['facebook_email_different'] ) ) {
		if ( isset( $_REQUEST['bemail'] ) ) {
			$_REQUEST['facebook_email'] = sanitize_email( stripslashes( $_REQUEST['bemail'] ) );
		} elseif ( is_user_logged_in() ) {
			$_REQUEST['facebook_email'] = $current_user->user_email;
		} else {
			$_REQUEST['facebook_email'] = '';
		}
	} elseif ( isset( $_REQUEST['facebook_email_different'] ) ) {
		$_REQUEST['facebook_email'] = sanitize_email( stripslashes( $_REQUEST['facebook_email'] ) );
	} else {
		$_REQUEST['facebook_email'] = '';
	}
	return true;
}
add_filter( 'pmpro_registration_checks', 'my_pmprorh_validate' );