$arr5 = [3 , 1];
$arr6 = [2 , 5, 7, 9];

$size1= count($arr5);
$size2 = count($arr5)+count($arr6);
$size3 = count($arr6);

for ($i = 0; $i <= $size2; $i++) {
    for ($j = $size1+1; $j <= $size2; $j++){
        for($k = 0; $k <= $size3; $k++ ){
        $temp = $arr6[$k];
        $arr5[$j] = $temp;
    }
    }
}

var_dump($arr5);