[FAQ] How do I pass PHP variables to javascript?

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JavaScript cannot actively read variables from the server, but when a page loads, PHP can pass variables to JavaScript by writing to the page the JavaScript is on.

 

 

<?php
   $username = "Snib";
?>
...
<script language="javascript" type="text/javascript">
   var sStr = "My name is <?php echo $username ?>.";

   document.write(sStr);
</script>
...

 

This code will output this text:

 

...

My name is Snib.

...

 

By editing the JavaScript on the page, PHP can set JavaScript variables.

This is very useful for passing database values to JavaScript to compare with a JavaScript variable or number. Let's say that there is a MySQL database called db1, with a table called table1, which has two columns, 'userid' and 'usernum'. To retrieve information about Snib, you can use code like this:

 

<?php

   $connection = mysql_connect('localhost', 'username', 'password');

   $sql = mysql_select_db('db1', $connection);

   $userid = "Snib";

   $query = "SELECT * FROM `table1` WHERE userid='$userid'";

   $result = mysql_query($query);

   $data = mysql_fetch_assoc($result);

   $usernum = $data['usernum'];

?>



<script language="javascript" type="text/javascript">

   var userid = "<?php echo $userid; ?>";

   var usernum = "<?php echo $usernum; ?>";

   //if usernum is less than or equal to 50
   if (usernum <= 50) { 

       alert("You are not allowed on this page, " + userid);
       setTimeout('window.location = "otherpage.php"', 3000); // user leaves in 3 seconds

   } else {

       alert('Welcome to this page, ' + userid);

   }

</script>

<?php mysql_close($connection); ?>

 

Although setting JavaScript variables with PHP is helpful, it cannot always be done. For instance, PHP cannot modify external .js documents temporarily. It changes them for good.

Reference:

MySQL with PHP: http://php.net/mysql

JavaScript: http://javascript.com http://msdn.microsoft.com/library/default.asp (JScript and DHTML)