<script>
var strWidth = document.getElementById("mydiv").style.width;
var strHeight = document.getElementById("mydiv").style.height;
var link = "<?php if(isset($_GET["ggg"])) {
echo $_GET["ggg"].".php?width=800&height=460";
} else {
echo "page1.php?width=800&height=460";
}
?>";
</script>

this is my script, php inside javascript. how do i place this variable strWidth inside

php?width=800&height=460

so becomes some how like this

php?width=strWidth&height=460

EDIT 2

well, the only thing i am trying to do here to show variable value between those line is it a big deal ?

it might be done by separating like using concatenation or something ?

Add an input-field in PHP, hide it (if necessary) and read the value of this field with JS.

you can't really do that. but this works

<?php
 echo "<script>
";    
 echo "var strWidth = document.getElementById(\"mydiv\").style.width;
";    
 echo "var strHeight = document.getElementById(\"mydiv\").style.height;
";    
 if(isset($_GET["ggg"])) {
       echo "var link =" . $_GET["ggg"] . ".php?width=800&height=460';
";    
 }
 else {
       echo "var link ='page1.php?width=' + strWidth + '&height=' + strHeight + '';
";    
 }
 echo "</script>
";    
?>

the reference to ggg completely confuses the understanding of this process so really it should be taken out:

__ page1.php
<?php

 if (!isset($_GET['foundWidth'])){
   //stops double hit
   echo "<script>
";    
   echo "var strWidth = document.getElementById(\"mydiv\").style.width;
";    
   echo "var strHeight = document.getElementById(\"mydiv\").style.height;
";    
   echo "var link ='/page1.php?foundWidth=true&width=' + strWidth + '&height=' + strHeight + '';
";
   echo "window.location = link;"
   echo "</script>
";    
 }
 elseif (isset($_GET['foundWidth']) && ($_GET['foundWidth']=='true')) {
     if (isset($_GET['width']) && is_numeric($_GET['width'])){
         //use your width here SERVER SIDE
         // or
         echo "<script> alert('Width is: '+ " . $_GET['width']) . "); </script>
";           
     }
     if (isset($_GET['height']) && is_numeric($_GET['height'])){
         //use your height here SERVER SIDE
         // or
         echo "<script> alert('Height is: '+ " . $_GET['height']) . "); </script>
";
     }
 }
?>

using this "trick" you can then write the PHP params into the javascript url with whatever get string you like, including triggering a reload of the page with the width as a param, so if you want to test if $_GET['width'] is set to a number you can insert it etc

First of all if you want to use php values in javascript you need the script to be written in a php file. Suppose you do this then you can do this in this way:

<script>
var strWidth = document.getElementById("mydiv").style.width;
var strHeight = document.getElementById("mydiv").style.height;
var link='<?php echo (isset($_GET["ggg"]))?isset($_GET["ggg"]):''; ?>'; // this assigns the valueto link variable
if(link==''){
// your logic starts here
}else{
// your logic starts here
}
</script>

Add an input-field and assign a value to the hidden element and then get value through javascript.

It is not a good idea to combine PHP and Javascript.

Refer this about explanation on client-side vs server-side coding.