I like the regular expression solutions but none of the others suit me.
If you know there is only gonna be 1 result you can use the following:
$between = preg_replace('/(.*)BEFORE(.*)AFTER(.*)/sm', '\2', $string);
Change BEFORE and AFTER to the desired delimiters.
Also keep in mind this function will return the whole string in case nothing matched.
This solution is multiline but you can play with the modifiers depending on your needs.