PHP bracket expression parser (experimental)

bracket.php

<?php
$str = "aaa(b, x(), c(d,e), f(g), h(i(3), j, k(l, n(z(0), 1), m)))";

/*
a->b
a->x
a->c
a->c
a->f
h->i
i->3
h->j
k->l
k->n
n->z
z->0
n->1
k->m
*/

$relations = array();

// step 1
$exps = array();
$chars = str_split($str);
$exp = "";
foreach($chars as $char) {
    if(in_array($char, array('(', ')', ','))) {
        $exp = trim($exp);
        if($exp != '') {
            $exps[] = trim($exp);
        }
        $exp = "";
        $exps[] = $char;
    } else {
        $exp .= $char;
    }
}

// step 2
$depth = 0;
$_exps = array();
$_depth = 0;
$_exp = "";
$_v_depth = 0;
$_v_exp = "";
foreach($exps as $exp) {
    if($exp == '(') {
        $depth++;
        array_push($_exps, $_exp);
    }

    if($exp == ')' && $_exp == '(') {
        $__exps = $_exps;
        $__exp_A = array_pop($__exps);
        $__exp_B = array_pop($__exps);
        $relations[] = array($__exp_B, $__exp_A);
    }

    if($exp == ')') {
        $depth--;
        array_pop($_exps);
    }

    // store previous expression
    if(!in_array($exp, array('(', ')'))) {
        if($_v_depth < $depth) {
            $v1 = current($_exps);
            while($v1 !== false) {
                $v2 = next($_exps);
                if($v2 !== false) {
                    $relations[] = array($v1, $v2);
                }
                $v1 = $v2;
            }
            $relations[] = array($_v_exp, $exp);
        }
        $_v_depth = $depth;
        $_v_exp = $exp;
    }

    $_depth = $depth;
    $_exp = $exp;
}

var_dump($relations);