MYSQL Fulltext Search Issue With Special Characters - php

Question

I am doing a fulltext search in PHP in BOOLEAN mode, which is working 95% of the time. However when I enter a special character like ", the query fails.
The table in question holds around 6,000 records, with a large quantity being commercial grade fasteners.
Example Data:
7/16" UNF x 2 1/4" Hex
Before the query is performed, the input is cleaned using the following function
function clean($db, $str) {
$str = #trim($str);
if(get_magic_quotes_gpc()) {
$str = stripslashes($str);
}
$str = htmlspecialchars($str, ENT_NOQUOTES);
$str = preg_replace("/[[:blank:]]+/"," ",$str);
return mysqli_real_escape_string($db, $str);
}
When searching for 7/16" and nothing else, I should get a few hundred results but I'm getting nothing and I believe it's to do with how the input is being escaped incorrectly.
Once the input is cleaned I add + to the beginning of each word to ensure it is included in the results. This is done using the following:
$symbol = '+';
$string = $symbol . str_replace(' ', " $symbol", $term);
Example Query:
SELECT * FROM products WHERE MATCH (code, desc) AGAINST ('+7\/16\\\"' IN BOOLEAN MODE) ORDER BY desc ASC
As you can see the original input of 7/16" has been changed to +7/16\\". Here is where I suspect the problem lies.
I'm really not sure what part of the clean function is causing this, I thought by adding ENT_NOQUOTES to htmlspecialchars would have resolved it but it hasn't.