Get Date From Form Using Post Method Script

<script>
function validate()
{
  if(document.frm.username.value=='')
  {
    alert("Please Enter Username");
    document.frm.username.focus()
    return false;
  }

  if(document.frm.email.value=='')
  {
    alert('Please Enter Email Address');
    document.frm.email.focus();
    return false;
  }

  var reg = /^([A-Za-z0-9_\-\.])+\@([A-Za-z0-9_\-\.])+\.([A-Za-z]{2,4})$/;
  var address = document.forms['frm'].elements['email'].value;
  if(reg.test(address) == false)
  {
    alert('This is invalid Email Address');
    document.frm.email.value='';
    document.frm.email.focus();
    return false;
  }
}
</script>

  <?php
  if(!isset($_POST[is_username]) && !isset($_POST[is_email]) && (count($_POST)!=0))
  {
    echo "<u>Display All Field and Its Value</u><br>";
    foreach ($_POST as $key => $value)
    {
      echo "$key = $value";
      echo "<br />";
    }
  }
?>

  <?php
  if(isset($_POST[is_username]) or isset($_POST[is_email]))
  {
    echo "<u>Display the Value of a Specific Field</u><br>";
    if(isset($_POST[is_username]) && isset($_POST[is_email]))
    {
      echo 'User Name :  '.$_POST[username]; echo "<br>";
      echo 'Email Address :  '.$_POST[email];
    }
  else if(isset($_POST[is_username]))
  {
    echo 'User Name :  '.$_POST[username];
  }
  else if(isset($_POST[is_email]))
  {
    echo 'Email Address :  '.$_POST[email];
  }
}
?>
<form id="frm" name="frm" method="post" onsubmit="return validate()">
User Name :  <input type="text" name="username" id="username" />
Email Address :   <input type="text" name="email" id="email" />
If you need to specific field value from POST method click at any one check box :   <input type="checkbox" name="is_username" id="is_username" /> User Name  <input type="checkbox" name="is_email" id="is_email"/> Email 
<input type="submit" name="submit" id="submit" value="Submit" /> 
</form>