Question

This is the var_dump($options)

array (size=4)
  0 => string '2' (length=1)
  'parent_2_children' => 
    array (size=3)
      0 => string '22' (length=2)
      1 => string '24' (length=2)
      2 => string '23' (length=2)
  1 => string '3' (length=1)
  'parent_3_children' => 
    array (size=3)
      0 => string '26' (length=2)
      1 => string '25' (length=2)
      2 => string '27' (length=2)

What I have tried up to now is

if(!is_null($options))
        {
            foreach($options as $option)
            {
                 if(!array_key_exists('parent_'.$option.'_children',$options))
                 {
                    //if position null output an error
                 }
            }   


        }

Print_r() as requested

Array
(
    [0] => 2
    [parent_2_children] => Array
        (
            [0] => 22
            [1] => 24
            [2] => 23
        )

    [1] => 3
    [parent_3_children] => Array
        (
            [0] => 26
            [1] => 25
            [2] => 27
        )

)

Answer 1

Your code is correct. An additional check on the nature of the key will reduce the execution, since you have to do the processing only for numeric keys.

    if(!is_null($options))
    {
        foreach($options as $key => $option)
        {
            if (is_numeric($key)) {
                 if(!array_key_exists('parent_'.$option.'_children',$options))
                  {
                       print 'parent_'.$option.'_children does not exist';
                   }
             }
        }   


    }

To test the code, try the following array :

 $options = array(0 => 2, 'parent_2_children' => array ( 0 => 22, 1 => 24, 2 => 23 ), 1 => 3, 'parent_3_children' => array ( 0 => 26, 1 => 25, 2 => 27 ), 2=>4 ); 

whose print_r output will be :

 Array
(
[0] => 2
[parent_2_children] => Array
    (
        [0] => 22
        [1] => 24
        [2] => 23
    )

[1] => 3
[parent_3_children] => Array
    (
        [0] => 26
        [1] => 25
        [2] => 27
    )

[2] => 4

)

and, it will output :

  parent_4_children does not exist