Should you want a similar function for splicing strings together, here is a rough equivalent:

<?php
function str_splice($input, $offset, $length=null, $splice='')
{
  $input = (string)$input;
  $splice = (string)$splice;
  $count = strlen($input);
  
  // Offset handling (negative values measure from end of string)
  if ($offset<0) $offset = $count + $offset;

  // Length handling (positive values measure from $offset; negative, from end of string; omitted = end of string)
  if (is_null($length)) $length = $count;
  elseif ($length < 0)  $length = $count-$offset+$length;

  // Do the splice
  return substr($input, 0, $offset) . $splice . substr($input, $offset+$length);
}

$string = "The fox jumped over the lazy dog.";

// Outputs "The quick brown fox jumped over the lazy dog."
echo str_splice($string, 4, 0, "quick brown ");

?>

Obviously this is not for cases where all you need to do is a simple search-and-replace.