为什么第二个PHP输入提交不起作用?

I have a form in my html page that runs a php page and show the results in real time on an Iframe, the operation is long about 20 minutes, I tried to implement a second button that stops the run killing the process, the php function works but the problem is that if I click the stop button the function doesn't start.Here the HTML:

 <div id="allign">
      <iframe id="frame" name="my_iframe"  height="520" width="1100" allign="right"  frameBorder="0" ></iframe>
 </div>


<form action="action_calibration.php" method=post  target="my_iframe">
<br><br>
  <input type="radio" name="type" value="fast" checked>Fast Calibration
<br><br>
  <input type="radio" name="type" value="old">Old Calibration
<br><br>
  <input type="checkbox" name="ScanVPlus" value="yes" checked>Scan Vcth vs VPlus
<br><br>
  <input type="checkbox" name="Bitwise" value="yes">Bitwise Offset Tuning
<br><br>
  <input type="checkbox" name="All_channel" value="yes">Calibration using all channel
<br><br>
Output Folder:<input type="textfield" name="output" value="Results/">
<br><br>
Hw Description File:<input type="textfield" name="Hw Description File " value="settings/Calibration2CBC.xml">
<br><br>
  <input type="submit" name="exec" value="Submit">
  <input type="submit" name="exec1" value="STOP">
<br><br>
</form> 

The final part of the action_calibration.php file is like this:

$handle = popen("source ".$command, "r");
$continue=TRUE;
while ((!feof($handle))&& $continue) {
    $data = fgets($handle);
    print $data."<br/>";
    echo $continue;
    if(isset($_POST['exec1'])){
     $continue=FALSE;
   }   
}
shell_exec("kill $(pidof calibrate)");

Thank you for your help.

try this ......

 <input type="submit" name="exec" value="Submit">
 <input type="submit" name="exec1" value="STOP">

 $handle = popen("source ".$command, "r");
 $continue=TRUE;
 while ((!feof($handle))&& $continue) {
 $data = fgets($handle);
 print $data."<br/>";
 echo $continue;
 if(isset($_POST['exec1'])){
 $continue=FALSE;
 }   
}
shell_exec("kill $(pidof calibrate)");