PHP program to exchange first and last characters in a string

I am trying do to this exercise:

“Write a PHP program to exchange the first and last characters in a given string and return the new string.”

The thing I’ve tried to do is this:

function stringReverse($str){

    $newStr = array($str);

    $tab = [];

    for($i = 0 ; $i<count($newStr); $i++){
        if($newStr[$i] == [0] && $str[$i] == [count($newStr)-1]){
            $tab[$i] = count($newStr)-1 && $tab[$i]= $newStr[0];
        }
        return $tab;
    }
}

echo stringReverse("abcd");

but it’s not working – I get this error :

PHP Notice: Array to string conversion in on line 89 Array

I’m expecting this result :

dbca

Goal of exercise : exchange first and last character of a string.

Can anyone help with this?