How to skip posts if the user is non-friend with a user?

I am struggling to solve this problem for days, and I can't figure out how should I do this.

I am able to get my own posts and the friends posts if both user are friends.

I want to check whether the user is friend with each other, and if not, don't show the non-friends posts.

The problem:

When the user friend with a user it's normally shows the friend's posts as well. The main issue is when

Error message: Notice: Undefined offset:

Example:

If the admin is friend with devel

The outcome:

Shows the admin's and devel's posts to each other correctly

If Zorro is friend with admin

The outcome:

Zorro's posts will be display to admin correctly, but it display the posts to devel as well with an error message Notice: Undefined offset:

If Zorro will become friend with devel

The error message will go away and will show Zorro's and Devel's posts to each other correctly.

The code to getting the posts.

Query:

 $sql = "SELECT * FROM posts JOIN connections WHERE posts.author = '$currentUser'  OR posts.author IN (SELECT FriendReferee FROM connections WHERE Friend ='$currentUser'
 or content<>'') OR posts.author IN
 (SELECT Friend FROM connections WHERE FriendReferee ='$currentUser'
 or content<>'') ORDER BY postID DESC";
 $result = $conn->query($sql);

Fetching the database:

  if ($result->num_rows > 0) {

      // output data of each row
  while($row = $result->fetch_assoc()) {

The loop:

    foreach($combine as $commentID => $imagesID) {
    if ($postedBy[$commentID] == $currentUser) {
       $profLink = '<a class="authorLink" href="../profile"><span class="author"><i class="fas fa-user-circle"></i> '.$postedBy[$commentID].'</span></a>';
     } else {
       $profLink = '<a class="authorLink" href="../view/?name='.$postedBy[$commentID].'"><span class="author">'.$postedBy[$commentID].'</span></a>';
     }
       echo $profLink;
       echo $comments[$commentID];
       if ($nameOfImage[$commentID] == NULL) {
        echo "";
      } else {
        foreach($imgs[$commentID] as $img) { // foreach loop that will render all the images...
          echo $img;
      }
      }
    }
  }
}