pmpro-email-subject-per-level.php

 <?php
/**
 * Change the email subject per template per level.
 *
 * 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 pmpro_email_subject_per_template_per_level( $subject, $email ) {
	$template = $email->template;

	if ( ! empty( $email->data['membership_id'] ) ) {
		$level_id              = $email->data['membership_id'];
		$membership_level_name = $email->data['membership_level_name'];
	} else {
		return $subject;
	}

	if ( strpos( $template, 'checkout' ) !== false ) {

		switch ( $level_id ) {
			case 1:
				if ( strpos( $template, '_admin' ) === false ) {
					$subject = 'Your Custom Subject for ' . $membership_level_name . ' Checkout - User';
				} else {
					$subject = 'Your Custom Subject for ' . $membership_level_name . ' - Admin';
				}
				break;
			case 2:
				if ( strpos( $template, '_admin' ) === false ) {
					$subject = 'Your Custom Subject for ' . $membership_level_name . ' Checkout - User';
				} else {
					$subject = 'Your Custom Subject for ' . $membership_level_name . ' - Admin';
				}
				break;
		}
	}

	return $subject;
}
add_filter( 'pmpro_email_subject', 'pmpro_email_subject_per_template_per_level', 10, 2 );