Remove a part of a string, but only when it is at the end of the string

I need to remove a substring of a string, but only when it is at the END of the string.

for example, removing 'string' at the end of the following strings :

"this is a test string" ->  "this is a test "
"this string is a test string" - > "this string is a test "
"this string is a test" -> "this string is a test"

Any idea's ? Probably some kind of preg_replace, but how??