pmpro-verify-minimum-required-age.php

<?php
/**
 * Verify that the user is at least 18 years old before registering.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */
function my_pmpro_min_required_age( $okay ) {

	$dob_field_name = 'birthday'; // The name of the user field that contains the date of birth.
	$min_age        = 18; // The minimum age required to register.

	// Bail if already not okay
	if ( ! $okay ) {
		return $okay;
	}

	// Get the birthday
	if ( isset( $_REQUEST[ $dob_field_name ] ) && ! empty( $_REQUEST[ $dob_field_name ] ) ) {
		$birthday = $_REQUEST[ $dob_field_name ];
	} else {
		pmpro_setMessage( 'Please fill out the ' . $dob_field_name . ' field.', 'pmpro_error' );
		return false;
	}

	// Check if the birthday values are numeric and is a valid date.
	if ( ! is_numeric( $birthday['m'] ) || ! is_numeric( $birthday['d'] ) || ! is_numeric( $birthday['y'] ) || ! checkdate( $birthday['m'], $birthday['d'], $birthday['y'] ) ) {
		pmpro_setMessage( 'Please enter a valid ' . $dob_field_name . '.', 'pmpro_error' );
		return false;
	}

	$now = new DateTime(); // Get the current date.
	$dob = new DateTime( $birthday['y'] . '-' . $birthday['m'] . '-' . $birthday['d'] ); // Get the date of birth.

	// Calculate the age of the user.
	$age = $dob->diff( $now )->y;

	// Check if the user is at least 18 years old.
	if ( $age < $min_age ) {
		pmpro_setMessage( 'You must be at least ' . $min_age . ' years old to register.', 'pmpro_error' );
		return false;
	}

	return $okay;
}
add_filter( 'pmpro_registration_checks', 'my_pmpro_min_required_age' );