Undefined variable problem with PHP function

This is because you're using the $pera variable (which exists only in the global scope) inside a function.

See the PHP manual page on variable scope for more information.

You could fix this by adding global $pera; within your function, although this isn't a particularly elegant approach, as global variables are shunned for reasons too detailed to go into here. As such, it would be better to accept $pera as an argument to your function as follows:

function provera($prom, $pera){
    if (preg_match("/[0-9\,\.\?\>\.<\"\'\:\;\[\]\}\{\/\!\\\@\#\$\%\^\&\*\(\)\-\_\=\+\`[:space:]]/",$prom)){
        echo "Nepravilan unos imena ili prezimina!";
        echo $pera;
        }
}

In your function function provera($prom) add a line that says

global $pera;

If your PHP version is on 5.3 or later versions, closure can be applied.

Closures may also inherit variables from the parent scope.

use is the php syntax to implement closure.

ref: Anonymous functions

    <?php
    // $ime=$_POST["ime"];
    // $prezime=$_POST["prezime"];
    $pera="string";
    $prezime = "Ne radi, vrati se nazad i unesi nesto!";
    // if (empty($ime)||empty($prezime)){
    //     echo "Ne radi, vrati se nazad i unesi nesto!";
    // }
    $provera = function ($prom) use ($pera) {
        if (preg_match("/[0-9\,\.\?\>\.<\"\'\:\;\[\]\}\{\/\!\\\@\#\$\%\^\&\*\(\)\-\_\=\+\`[:space:]]/",$prom)){
            echo "Nepravilan unos imena ili prezimina!";
            echo $pera;
        }
    };

    // $provera($ime);
    $provera($prezime);