Try something like this:

$pattern = "/[^A-Za-z0-9\-_.?=&:+']/";
$string = preg_replace($pattern,'',$string);

Refer to pattern syntax [php.net] for more information :)

thnx eelixduppy, however i wanted the opposite of that in other words i would like to keep A-Za-z0-9\-_.?=&:+' but strip out the rest.

Have you tried the script? That is exactly what it does ;)

Just a clarification. Note ^ just after the [
it means "not" when included as first character of the set [].