PLEASE how to echo a variable within another variable

[pixeldesigner]

So I'm fairly new to php and just need some quick help. Here is my code:

 

function plaintext_category(){
	$cat_plain = strip_tags( get_the_term_list($post->ID, 'portfolio_category', '', ', ', '' ) );
	$cat_plain = strtolower($cat_plain);
	$cat_plain = str_replace(' ','-',$cat_plain);
	echo $cat_plain;

}

$pattern = '/title=\"(.*?)\"/';
$replace = 'title="echo $cat_plain"'; /* This is where i need help, how do i echo this? */
$categories = preg_replace($pattern,$replace,$categories);
echo $categories;

 

So I'm trying to pass the echo value of $cat_plain but when I put echo in front of it, i get an error. If I don't put echo, I just get a blank result.

 

Please, need some help on this. Thanks guys!

[litebearer]

here is a start...

http://php.net/manual/en/language.variables.scope.php