Unable to access local variables in for, php

I have a little piece of code that seems to be working fine and dandy, but i cant acces the local variable from the for function. I also tried it with while and still no success. What am I doing wrong?

$numbers = array(57, 70, 83, 96, 109, 122, 135);

$content = [];
for($i = 0; $i <= count($numbers)-1; $i++){

    if(!in_array(getGroupAttributes($numbers[$i]), $content)){
        $attr = getGroupAttributes($numbers[$i]);
        $content = array_merge($content, getGroupAttributes($numbers[$i]));
        var_dump($content);
    }

}

The content array is always empty. The $numbers array seems to be accesible.