How do you use php variables in css properly?

You can't render PHP inside of a .css file. You should use inline styles in your HTML that's generated by a PHP file.

That is the direct answer to your question. The indirect answer:

Use SASS or LESS. They're designed for doing exactly what you want here: using variables in your CSS.

You can do use it like this, inline in the HTML:

 <style>
-webkit-clip-path: inset(0 1500px ("<?php echo $y; ?>") 0);
-moz-clip-path: inset(0 1500px ("<?php echo $y; ?>") 0);
-o-clip-path: inset(0 1500px ("<?php echo $y; ?>") 0);
clip-path: inset(0 1500px ("<?php echo $y; ?>") 0);
</style>

Rename your style.css to style.php and linking it with your html file like this

<link rel='stylesheet' type='text/css' href='css/style.php' />

At the top of your new style.php file set the Content-type back to CSS:

<?php
header("Content-type: text/css; charset: UTF-8");
?>

Now you are good to go... Set up and use your variables

You can :
1) Insert the value in the inline CSS directly (refer How do I dynamically insert a CSS class using PHP?)
2) Insert the value from PHP into CSS via jQuery (refer Add CSS class based on ID)

P.S. : Search for existing question-answers before you post