<? set_time_limit(0);
   
   //first possible bug
   $s="Hello";
   $ss="$s world!";
   echo "1 $ss\n";  //echo as expected "Hello world!"
   $ss='$s world!'; //the only difference from previous                                         //assigment is quotation marks
   echo "2 $ss\n";  //a bit different output "$s world!"
   
   //second bug eval()
   $cmd="echo '$s world!\n';";
   
   for($i=0;1;$i++)
    { echo "$i:";
      eval($cmd);
    }  
?>

Script output
// first bug
1 Hello world!
2 $s world!

//second bug with eval()
4094:Hello world!
4095:<br>
<b>Fatal error</b>:  Unable to initialize a new token cache in <b>bug.php3</b> on line <b>16</b><br>