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I am trying to pass an array of category IDs to get_category_by_ID() in order to get an array of category names back. Since I'm still trying to get this to work I'm just using var_dump() on the output. I'm using the values 1, 5, 3 since 5 and 3 are categories I have and 1 is uncategorized, which I need this to work for.

I started with this:

    $CatID = array(1, 5, 3,);
foreach ($CatID as $ID) { 
    $names = get_the_category_by_ID($ID);
}
var_dump($names);

I returned the correct category name for category 3, but nothing else. So then I tried it like this:

$CatID = array(1, 5, 3,);
$x=0;
foreach ($CatID as $ID) { 
    if ($x==3) break;
    $names[$x] = get_the_category_by_ID($ID[$x]);
    $x++;
}
var_dump($names);

This time I got this array of errors:

array
0 => 
object(WP_Error)[262]
  public 'errors' => 
    array
      'invalid_term' => 
        array
          ...
  public 'error_data' => 
    array
      empty
 1 => 
 object(WP_Error)[261]
  public 'errors' => 
    array
      'invalid_term' => 
        array
          ...
  public 'error_data' => 
    array
      empty
2 => 
object(WP_Error)[215]
  public 'errors' => 
    array
      'invalid_term' => 
        array
          ...
  public 'error_data' => 
    array
      empty
1

Modify your first code chunk thusly:

$names = array();
$CatID = array( 1, 5, 3,);
foreach ( $CatID as $ID ) { 
    $names[] = get_the_category_by_ID( $ID );
    // uncomment the next line to get the name tied to the ID
    // $names[$ID] = get_the_category_by_ID( $ID );
}
var_dump( $names );

See PHP's array() reference for more info on using arrays.

Why your code isn't working

In your first example, you're looping through your $CatID array, assigning the return from get_the_category_by_ID() to $names. Each time through the loop, you're overwriting the $names variable, which is why only 3 -- the last one in your list -- is sticking.

In your second example, you're swinging too far the other way -- you're trying to get the value of $ID[$x], but $ID isn't an array, so you're handing an invalid value (probably null) to get_the_category_by_ID(). That's why it's returning a passel of WP_Error objects.

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