Why not escape it the way it is meant to be done
$search='r/trtrt';
echo preg_replace('/\b'.preg_quote($search, '/').'\b/', 'ERTY', 'TG FRT');
http://php.net/manual/en/function.preg-quote.php
preg_quote() takes str and puts a backslash in front of every
character that is part of the regular expression syntax. This is
useful if you have a run-time string that you need to match in some
text and the string may contain special regex characters
delimiter
If the optional delimiter is specified, it will also be escaped.
This is useful for escaping the delimiter that is required by the PCRE
functions. The / is the most commonly used delimiter.
Add slashes is not the function to use here. It provides no escaping for any of the special characters in Regx.
The special regular expression characters are: . \ + * ? [ ^ ] $ ( ) {
} = ! < > | : -
Using the proper functions promote readability of the code, if at some later point in time you or another coder see the ~ delimiter they may just think its part of a personal "style" or pay it little attention. However, seeing the input properly escaped will tell any experienced coder that the input could contain characters that conflict with regular expressions.
Personally, readability is at the top of my list whenever I write code. If you cant understand it at a glance, what good is it.