Check on the PHP side

Based on the present code in the question, the simplest is to check the content of the variables with PHP, thus preparing the value to be used by JavaScript on the client side:

if (is_null($string_patr)) {
    $vv_patr = $string_seri;
}
else {
    $vv_patr = $string_patr.','.$string_seri;
}

echo '
<script type="text/javascript">
  var vv_patr = "'.$vv_patr.'";
  alert(vv_patr);
</script>';

Example on Ideone .

In this way PHP checks the variables and when it is to send the JavaScript code to the browser, it goes less code and is ready to be used.

Learn more about PHP's is_null() function.

Check on the JavaScript side

To keep your code with logic on the JavaScript side, you'll need to change the way you check the contents of the variable.

The null in the PHP variable will reflect nothing in the JavaScript variable.

$string_patr = null;
$string_seri = "bubu";

print("<SCRIPT language=javascript>
string_patr = \"$string_patr\";
string_seri = \"$string_seri\";
if(string_patr == null){
vv_patr = string_seri;
}
else{
vv_patr = string_patr+','+ string_seri;
}
alert(vv_patr); 
</SCRIPT>");

Will result in:

<SCRIPT language=javascript> 
    string_patr = "";
    string_seri = "bubu";
    if(string_patr == null){
        vv_patr = string_seri;
    }
    else{
        vv_patr = string_patr+','+ string_seri;
    }
    alert(vv_patr);
</SCRIPT>

Notice that null on the PHP side turned out to be nothing on the JavaScript side. Since you have double quotation marks, you end up with string_patr = ""; which allows you to change the check to:

 if ((!string_patr || string_patr.length === 0) {
    vv_patr = string_seri;
 }
 else {
    vv_patr = string_patr+','+ string_seri;
 }

So we're seeing if the variable actually exists and is not empty.

Note: The type of verification depends on what is expected to be in the variable, in the suggested check above you are checking whether it is null , undefined or empty. p>     

Try this first:

<?php
    echo "<SCRIPT> 
                string_patr = ".$string_patr.";
                string_seri = ".$string_seri.";
                if(string_patr == null){
                    vv_patr = string_seri;
                }
                else{
                    vv_patr = string_patr+','+ string_seri;
                }
                alert(vv_patr);
    </SCRIPT>";
?>

<script language=javascript> 

   function isNullCustom(test) 
   {
       return (test == null || test == "null" || typeof(test) == "undefined" || test == "");
   }

   var string_patr = "<?php echo $string_patr; ?>";
   var string_seri = "<?php echo $string_seri; ?>";

   if(isNullCustom(string_patr))
   {
      vv_patr = string_seri;
   }
   else
   {
      vv_patr = string_patr+','+string_seri;
   }

   alert(vv_patr);

</script>

I think the problem is not in php. When you print null in a javascript variable, it is not a boolean value but a string with the text ' null / strong> '.

I created a custom function to identify possible null cases for what you want.

Where, a variable that looks like:

• Boolean null ;
• string null ;
• empty string (0 characters);
• or with its undefined type;

They will be considered null.

See if it works.