$message = strip_tags( $message); should have shown $message = strip_tags('{{message}}');, I mistyped.

I’ve edit the code in class-fep-email-beautify.php to allow me to do some testing (I will change it back once I have a solution), which means I’ve been able to pull the message into the email template without the <p></p>.

But even thought I have {{message}} in a $string, I’m still unable to modify or validate it using any sort of PHP function. It seems immune to my tinkering.

This lot works as soon as I switch out {{message}} for the actual message copy, until then it comes back ‘false’ even when I know if should be ‘true’.

function rip_tags($string, $rep = ' ') { 
    
    $string = preg_replace ('/<[^>]*>/', $rep, $string); 
    $string = str_replace("\r", '', $string);    // --- replace with empty space
    $string = str_replace("\n", $rep, $string);   // --- replace with space
    $string = str_replace("\t", $rep, $string);   // --- replace with space
    $string = trim(preg_replace('/ {2,}/', $rep, $string));
    
    return $string; 

}

/* strpos that takes an array of values to match against a string */
function strpos_arr($haystack, $needle) {

	foreach($needle as $value) {
		if (strpos($haystack, $value) !== false) {
			return "<p><b>Warning!</b></p>";
		}
	}
	return false;
	
}

$message = "{{message}}";
$message = rip_tags($message);
$needle = array('various scam', 'keywords', 'and other stuff');
$haystack = $message;

I got it all working, I found the reference I need to understand the filter here.

Here’s the working code.

function fep_rip_tags($string, $rep = ' ') { 

    $string = preg_replace ('/<[^>]*>/', $rep, $string); 
    $string = str_replace("\r", '', $string);    // --- replace with empty space
    $string = str_replace("\n", $rep, $string);   // --- replace with space
    $string = str_replace("\t", $rep, $string);   // --- replace with space
    $string = trim(preg_replace('/ {2,}/', $rep, $string));
    return $string; 

}

function fep_strpos_arr($haystack) {
	
	$needle = array('a selection', 'of various', 'keywords', 'related to scams');
	foreach($needle as $value) {
		if (strpos($haystack, $value) !== false) {
			return "<p><b>Warning!</b> This is a scam.</p>";
		}
	}
	return false;

}

add_filter( 'fep_filter_message_before_send', function( $message ) {

	$haystack = $message['message_content'];
	$message['message_content'] = fep_strpos_arr($haystack) . fep_rip_tags( $message['message_content'] );
	return $message;
	
}, 99);

It was the $message['message_content']; bit that I was unable to find in the plugin code or docs. But now I know what to look for, it’s everywhere. I’ve return my class-fep-email-beautify.php back to it’s default state too, reenabling wpautop on the copy.