Eval in PHP and security measures , Creating a PHP demo editor

I know that eval is the function in PHP to execute PHP code from an input. Now I want to make a W3Schools like editor. What can I do to protect eval code that I get from POST variable.

$code = eval($_POST["phpusercode"]);
echo $code;

What I want to do is when a user will make a function like this

I want to give user the ability to write his own PHP code on my site without making my website vulnerable to some sort of hacking.

Solution

eval evaluates code, so, as @sectus says in comments, execute the code

For example:

eval ("echo 'Hello user'"); //This will execute echo 'Hello user'

So, in your case i think you don't want to execute your user code, so please carify your question and update it.

IMPORTANT:

Use of eval is highly discouraged
NEVER EVER use eval with params by POST/GET without sanitize them

Useful links:

When eval is evil

Avoid SQL injection