shopt -s histappend
PROMPT_COMMAND="history -a;$PROMPT_COMMAND"

$array = array(
  'one' => 'hi',
  'two' => 'privet',
  'three' => array('three_1' => 'hi', 'three_2' => 'privet'),
);

$str = 'three|three_1';

$keys = explode("|", $str);

var_dump($array[$keys[0]][$keys[1]]);

<?php

function foo($array, $str){
	$keys = explode('|', $str);
	
	$keysCount = count($keys);
	
	$result = $array;
	for($i = 0; $i < $keysCount; $i++){
		if(!is_array($result) && $i < ($keysCount - 1)){
			throw new Exception("Array depth not expected");
		}
		
		if(!isset($result[$keys[$i]])){
			throw new Exception(sprintf("Key '%s' not found", $keys[$i]));
		}
		
		$result = $result[$keys[$i]];
	}
	
	return $result;
}

$array = array(
  'one' => 'hi',
  'two' => 'privet',
  'three' => array('three_1' => 'hi', 'three_2' => 'privet'),
);

$str = 'three|three_1';

var_dump(foo($array, $str));