Question

I want to insert a element inside a array, but not overwrite any existing elements:

$to_insert = 25;
$elem  = 'new';

$arr = array(
  5 => 'abc',
 10 => 'def',
 12 => 'xyz', 
 25 => 'dontoverwrite',
 30 => 'fff',
);

foreach($arr as $index => $val){
  if($to_insert == $index){
    // here get next free index, in this case would be 26;
    $arr[$the_free_index] = $elem;
  }   
}

How can I do that?

Answer 1

You want a simple loop that starts from $to_insert and increases the loop variable until it finds a value that does not already exist as a key in $arr. So you can use for and array_key_exists:

for($i = $to_insert; array_key_exists($i, $arr); ++$i) ;
$arr[$i] = $elem;

This will correctly insert the element both when the $to_insert key exists and when it does not.

See it in action.