I was working with MySQL and displaying the title to things on the web page.  I'd written a script to ensure single and double quotes were removed from the title.  I used

    $title = str_replace( "'", "", $title );

and

     $title = str_replace( '"', "", $title );

But still the single and double quotes continued.  So I wrote a bit of code to print out each character separated by a dash.  Like so:

     for( $i=0; $i<strlen($title); $i++ ){
         echo "$i-";
         }

     echo "<br>\n";

This displayed:

     m-y-c-o-m-p-a-n-y- b-b-&-#-3-9-;-s

Which made me go "Oh!  I get it."

The MySQL function real_escape_string modifies the single quotes to be &#39; and double quotes as &#34;  These still show up as single and double quotes under HTML and most importantly - 

     JAVASCRIPT sees the &#34; and &#39; as actual single or double
     quotes.  So if you are passing arguments to a function you have
     to get rid of them or else you will get an error on trying to call
     a given function.  Example:

     <a href="javascript:func1('mycompany bbs&#39;s")'">

     becomes

     <a href="javascript:func1('mycompany bbs's');">

Which then will give you an error because there is a single quote inside of the single quoted string.  HOWEVER, the

     $title = str_replace( "'", "", $title );

WILL NOT FIND a single quote.  Instead, you have to do this:

     $title = str_replace( "&#39;", "'", $title );
and
     $title = str_relace( "&#34;", '"', $title );

(Or you could just get rid of them.)

So remember!  If you are trying to remove single and double quotes and are using MySQL and MySQL's real_escape_string() function that you might be having single and double quotes hanging around which are defined as &#39; and &#34; but which show up as single and double quotes as well as causing problems in your Javascripts.