<pre><?php
/** get leading, trailing, and embedded separator tokens that were 'skipped'
if for some ungodly reason you are using php to implement a simple parser that 
needs to detect nested clauses as it builds a parse tree */

$str = "(((alpha(beta))(gamma))";

$seps = '()';
$tok = strtok( $str,$seps ); // return false on empty string or null
$cur = 0;      
$dumbDone = FALSE;
$done = (FALSE===$tok);
while (!$done) {
   // process skipped tokens (if any at first iteration) (special for last)
   $posTok = $dumbDone ? strlen($str) : strpos($str, $tok, $cur );
   $skippedMany = substr( $str, $cur, $posTok-$cur ); // false when 0 width
   $lenSkipped = strlen($skippedMany); // 0 when false
   if (0!==$lenSkipped) {
      $last = strlen($skippedMany) -1;
      for($i=0; $i<=$last; $i++){
         $skipped = $skippedMany[$i];
         $cur += strlen($skipped);
         echo "skipped: $skipped\n";
      }
   }
   if ($dumbDone) break; // this is the only place the loop is terminated

   // process current tok
   echo "curr tok: ".$tok."\n";

   // update cursor
   $cur += strlen($tok);

   // get any next tok
   if (!$dumbDone){
      $tok = strtok($seps);
      $dumbDone = (FALSE===$tok); 
      // you're not really done till you check for trailing skipped
   }
};
?></pre>