$post = Array(1 => 3,5 => 6,7 => 4);
$bas = Array (1 => 1,5 => 1,7 => 1);

foreach($post as $key => $val){
    $s = array_search($key,$bas);
    if($s !== false){
         $bas[$s] = $val;
    }
    
}
print_r($bas);

$post = Array(1 => 3,5 => 6,7 => 4);
$bas = Array (1 => 1,5 => 1,7 => 1);

foreach($bas as $k=>$v)
    if(isset($bas[$k]))
        $post[$k] = $bas[$k]; 
		

echo '<pre>' . print_r($post, 1) . '</pre>';