Here is a sequence of code which I found doesn't work.
I have a better way of performing this function with regexp's but this should work without crashing anyway.

I have had a play with this function removing bits here and there to find out exactly where the problem is.
It seems that if you remove this section.
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      case "<":
        $stack++;
        break;
      case ">":
        $stack--;
        break;
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There is no crash.
Also if the for loop is removed there is no crash.
Also if I replace the switch statement line with this
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    $temp = $str[$i];
    switch($temp) {
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It works too.

Thought that might make it a bit easier to track down.

Here is the function below.
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function StripHTML($str) {
  $dest = "";
  $stack = 0;
  for ($i=0;$i<strlen($str);$i++) {
    switch($str[$i]) {
      case "<":
        $stack++;
        break;
      case ">":
        $stack--;
        break;
      default:
        if (!$stack) {
          $dest .= $str[$i];
        }
        break;
    }
  }
  return $dest;
}
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