How to detect if a string contains PHP code? PHP

I am keeping record of every request made to my website. I am very aware of the security measurements that need to be taken before executing any MySQL query that contains data coming from query strings. I clean it as much as possible from injections and so far all tests have been successful using:

htmlspecialchars, strip_tags, mysqli_real_escape_string.

But on the logs of pages visited I find query strings of failed hack attempts that contain a lot of php code:

?1=%40ini_set%28"display_errors"%2C"0"%29%3B%40set_time_limit%280%29%3B%40set_magic_quotes_runtime%280%29%3Becho%20%27->%7C%27%3Bfile_put_contents%28%24_SERVER%5B%27DOCUMENT_ROOT%27%5D.%27/webconfig.txt.php%27%2Cbase64_decode%28%27PD9waHAgZXZhb

In the previous example we can see:

display_errors, set_time_limit, set_magic_quotes_runtime, file_put_contents

Another example:

/?s=/index/%5Cthink%5Capp/invokefunction&function=call_user_func_array&vars[0]=file_put_contents&vars[1][]=ctlpy.php&vars[1][]=<?php @assert($_REQUEST["ysy"]);?>ysydjsjxbei37$

This one is worst, there is even some <?php and $_REQUEST["ysy"] stuff in there. Although I am able to sanitize it, strip tags and encode < or > when I decode the string I can see the type of requests that are being sent.

Is there any way to detect a string that contains php code like:

filter_var($var, FILTER_SANITIZE_PHP); 

FYI: This is not a real function, I am trying to give an idea of what I am looking for.

or some sort of function:

function findCode($var){
   return ($var contains PHP) ? true : false
}

Again, not real

No need to sanitize, that has been taken care of, just to detect PHP code in a string. I need this because I want to detect them and save them in other logs.

NOTE: NEVER EXECUTE OR EVAL CODE COMING FROM QUERY STRINGS

After reading lots of comments @KIKO Software came up with an ingenious idea by using PHP tokenizer, but it ended up being extremely difficult because the string that is to be analyzed needed to have almost prefect syntax or it would fail.

So the best solution that I came up with is a simple function that tries to find commonly used PHP statements, In my case, especially on query strings with code injection. Another advantage of this solution is that we can modify and add to the list as many PHP statements as we want. Keep in mind that making the list bigger will considerably slow down your script. this functions uses strpos instead of preg_match (regex ) as its proven to perform faster.

This will not find 100% PHP code inside a string, but you can customize it to find as much as is required, never include terms that could be used in regular English, like 'echo' or 'if'

function findInStr($string, $findarray){
    $found=false;   
    for($i=0;$i<sizeof($findarray);$i++){
        $res=strpos($string,$findarray[$i]);
        if($res !== false){
            $found=true;
            break;
        }
    }
    return $found;
}

Simply use:

$search_line=array(
            'file_put_contents',
            '<?=',
            '<?php',
            '?>',
            'eval(',
            '$_REQUEST',
            '$_POST',
            '$_GET',
            '$_SESSION',
            '$_SERVER',
            'exec(',
            'shell_exec(',
            'invokefunction',
            'call_user_func_array',
            'display_errors',
            'ini_set',
            'set_time_limit',
            'set_magic_quotes_runtime',
            'DOCUMENT_ROOT',
            'include(',
            'include_once(',
            'require(',
            'require_once(',
            'base64_decode',
            'file_get_contents',
            'sizeof',
            'array('
        );

if(findInStr("this has some <?php echo 'PHP CODE' ?>",$search_line)){
  echo "PHP found";
}