Code:
<?php
$widthVariable=200;
?>
<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <link href="widthVariable.css" rel="stylesheet" type="text/css">
    <title>widthVariable</title>
<style type="text/css"> 
.widthVariable {width:<?php echo $widthVariable ?>px;background-color:yellow}
</style>
</head>  
<body>
<div class="widthVariable">
myDiv
</div>
</body>
</html>
In the code above, I can change the width of div by changing the $widthVariable.

I like to remove the part below in the page above.
Code:
<style type="text/css"> 
.widthVariable {width:<?php echo $widthVariable ?>px;background-color:yellow}
</style>
Instead of removing the code abvoe, I like to put the code below into the widthVariable.css.

Code:
.widthVariable {width:<?php echo $widthVariable ?>px;background-color:yellow}
But the green part of the code in widthVariable.css doesn't work because it's PHP code in css file.

Is there any way to make php code in a css file do work ?