well. if you want to put quotes in the str_replace() function, i'd use single quotes (or double quotes but you have to escape the quote)
if i'm not mistaken you want to replace double quotes ("") to a single (") quote.
examples (both ways should work equally)
str_replace('""', '"', $text);
str_replace("\"\"", "\"", $text); // you have to escape the quote.
if you are using this method to ouput the text to HTML, you should look into function htmlspecialchars(). it basically converts quotes (") into an HTML entity ("😉
just for reference, here are differences between single and double quote usage:
http://www.php.net/manual/en/language.types.string.php
http://us3.php.net/echo
hope it was helpful.
sijis