Use PCRE Recursive pattern to match substrings in nested parentheses:

$str = "a(bc)de(fg)h some text a(bcd(ef)g)h ";
preg_match_all("/\((((?>[^()]+)|(?R))*)\)/", $str, $m);

print_r($m[1]);

The output:

Array
(
    [0] => bc
    [1] => fg
    [2] => bcd(ef)g
)

\( ( (?>[^()]+) | (?R) )* \)

First it matches an opening parenthesis. Then it matches any number of substrings which can either be a sequence of non-parentheses, or a recursive match of the pattern itself (i.e. a correctly parenthesized substring). Finally, there is a closing parenthesis.

Technical cautions:

If there are more than 15 capturing parentheses in a pattern, PCRE has to obtain extra memory to store data during a recursion, which it does by using pcre_malloc, freeing it via pcre_free afterwards. If no memory can be obtained, it saves data for the first 15 capturing parentheses only, as there is no way to give an out-of-memory error from within a recursion.

This question pretty much has the answer, but the implementations are a little ambiguous. You can use the logic in the accepted answer without the ~s to get this regex:

\(((?:\[^\(\)\]++|(?R))*)\)

Tested with this output:

Please can you try that:

preg_match("/\((.+)\)/", $input_line, $output_array);

Test this code in http://www.phpliveregex.com/

Regex: \((.+)\)
Input: a(bcd(eaerga(er)gaergf)g)h
Output: array(2
   0    =>  (bcd(eaerga(er)gaergf)g)
   1    =>  bcd(eaerga(er)gaergf)g
)