Brainfuck interpreter in PHP

<?
$instructionpointer = 0;
$memorypointer = 0;
$memory = array();
$callstack = array();
// clear the memory cell array
for ($i = 0; $i < 1000; $i++) {
  $memory[$i] = 0;
}
$brainfuckcode = '++++++++++[>+++++++>++++++++++>+++>+<<<<-]>++.>+.+++++++..+++.>++.<<+++++++++++++++.>.+++.------.--------.>+.>.';
$length = strlen($brainfuckcode);
while ($instructionpointer < $length) {
  $opcode = substr($brainfuckcode,$instructionpointer,1);
  switch ($opcode) {
    case "." :
      echo chr($memory[$memorypointer]);
      break;
    case "+" :
      $memory[$memorypointer]++;
      break;
    case "-" :
      $memory[$memorypointer]--;
      break;
    case ">" :
      $memorypointer++;
      break;
    case "<" :
      $memorypointer--;
      break;
    case "[" :
      $callstack[] = $instructionpointer;
      break;
    case "]" :
      if ($memory[$memorypointer] != 0) {
        $instructionpointer = $callstack[count($callstack)-1];
      } else {
        array_pop($callstack);
      }
      break;
  }
  $instructionpointer++;
}
?>