get_birthday_at_checkout.php

<?php
/*
	Add birthday (month and day only) to user signup.
*/

// Add birthday fields at checkout.
function my_pmpro_birthday_checkout_after_email() {
	$current_year = date("Y");
	?>
	<div>
		<label for="birthday_month">Birthday</label>
		<select name="birthday_month" required>
			<option value="">Month</option>
			<?php
			for ( $i = 1; $i <= 12; $i++ ) {
				$selected = ( ! empty( $_REQUEST['birthday_month'] ) && intval( $_REQUEST['birthday_month'] ) === $i ) ? 'selected' : '';
				echo '<option value="' . esc_attr( $i ) . '" ' . $selected . '>' . esc_html( date( "M", strtotime( $i . "/15/" . $current_year ) ) ) . '</option>';
			}
			?>
		</select>
		<input type="number" name="birthday_day" min="1" max="31" placeholder="Day" value="<?php echo ! empty( $_REQUEST['birthday_day'] ) ? intval( $_REQUEST['birthday_day'] ) : ''; ?>" required />
		<small>month/day</small>
	</div>
	<?php
}
add_action( 'pmpro_checkout_boxes', 'my_pmpro_birthday_checkout_after_email' );

// Validate birthday input.
function my_pmpro_birthday_registration_checks( $okay ) {
	if ( ! $okay ) {
		return $okay;
	}

	global $pmpro_msg, $pmpro_msgt;

	$birthday_month = intval( $_REQUEST['birthday_month'] );
	$birthday_day   = intval( $_REQUEST['birthday_day'] );

	if ( empty( $birthday_month ) || empty( $birthday_day ) ) {
		$pmpro_msg  = 'Please enter your birth month and day.';
		$pmpro_msgt = 'pmpro_error';
		return false;
	}

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

// Save birthday to user meta.
function my_pmpro_birthday_after_checkout( $user_id ) {
	if ( ! empty( $_REQUEST['birthday_month'] ) && ! empty( $_REQUEST['birthday_day'] ) ) {
		$birthday_month = str_pad( intval( $_REQUEST['birthday_month'] ), 2, '0', STR_PAD_LEFT );
		$birthday_day   = str_pad( intval( $_REQUEST['birthday_day'] ), 2, '0', STR_PAD_LEFT );

		// Store in MM-DD format.
		$birthday = $birthday_month . '-' . $birthday_day;
		update_user_meta( $user_id, 'birthday', $birthday );
	}
}
add_action( 'pmpro_after_checkout', 'my_pmpro_birthday_after_checkout' );

// Add birthday to Members List columns.
function my_pmpro_members_list_heading_birthday( $columns ) {
	$columns['birthday'] = 'Birthday';
	return $columns;
}
add_filter( 'pmpro_memberslist_extra_cols', 'my_pmpro_members_list_heading_birthday' );

// Display the stored birthday.
function my_pmpro_members_list_user_birthday( $user ) {
	$birthday = get_user_meta( $user->ID, 'birthday', true );
	$user->birthday = ! empty( $birthday ) ? esc_html( $birthday ) : '';
	return $user;
}
add_filter( 'pmpro_members_list_user', 'my_pmpro_members_list_user_birthday' );