How to use array_filter in loop

First of all: I recommend you not to use $listing along with $listings as variable names as it can easily lead to confusion and is not a good readability (especially confusing here on StackOverflow).

Then: You have an error in your code. You are not checking for the length (count) but for the string itself which does resolve in TRUE.

You have:

$NUM = 2;

$listings[$i] = array_filter($listings[$i], function ($element){ 
    return ($element[$i] > $NUM);
    }
); 

You should have:

$NUM = 2;

$listings[$i] = array_filter($listings[$i], function ($element) use ($NUM) { 
    return (strlen($element[$i]) >= $NUM);
    }
); 

You may try the following code:

function arr_filter($arr, $min_length) {
    define('STR_MIN_LEN', $min_length);
    $arr = array_filter($arr);
    $arr_explode = [];

    foreach($arr as $value) {
        $arr_explode = explode(',', $value);
        $arr_explode = array_unique(array_filter($arr_explode));
        $arr_explode = array_filter($arr_explode, function($element) {
            return strlen($element) >= STR_MIN_LEN;
        });
    }

    return array_values(array_unique($arr_explode));
}

var_dump(arr_filter($arr, 2));

The above code is written as per your requirements. You could try it out and see if it works. This code may not be flexible. You can check it out with various test cases. Hope it works.

EDIT

I assume you have a multidimensional array like:

$arr = array(
    array(
        'Chocolate Manufacturers,C',
        ',,,Chocolate Manufacturers,,Chocolate Manufacturers,,Chocolate Manufacturers',
        ''
    ),
    array(
        'Ice Cream Manufacturers,C',
        ',,,Ice Cream Manufacturers,,Ice Cream Manufacturers,,Ice Cream Manufacturers',
        ''
    )
);

and the code is:

function arr_filter($arr, $min_length) {
    define('STR_MIN_LEN', $min_length);
    $final_arr = array();

    foreach($arr as $value) {
        $value = array_filter($value);
        $arr_explode = [];

        foreach($value as $another_value) {
            $arr_explode = explode(',', $another_value);
            $arr_explode = array_unique(array_filter($arr_explode));
            $arr_explode = array_filter($arr_explode, function($element) {
                return strlen($element) >= STR_MIN_LEN;
            });
        }

        $final_arr[] = array_values(array_unique($arr_explode));
    }

    return $final_arr;
}

var_dump(arr_filter($arr, 2));

I've added this code to make it work with multidimensional array. Hope it works for sure.

EDIT 2

function arr_filter($arr, $min_length) {
    define('STR_MIN_LEN', $min_length);
    $final_arr = array();

    foreach($arr as $value) {
        $value = array_filter($value);
        $arr_explode_final = [];

        foreach($value as $another_value) {
            $arr_explode = explode(',', $another_value);
            $arr_explode = array_filter($arr_explode, function($element) {
                return strlen($element) >= STR_MIN_LEN;
            });

            // the comma will be added if the string has two different words with comma-separated like `Chocolate Manufacturers, Ice Cream Manufacturers` else comma will be ommited
            $arr_explode_final[] = implode(',', array_unique($arr_explode)); 
        }

        $final_arr[] = $arr_explode_final;
    }
    return $final_arr;
}

var_dump(arr_filter($arr, 2));

As you are code say, $listings is contains sentence. If I got your problem properly then, you want to remove smaller from each sentences of the $listings variable. You can replace your this code:

$NUM = 2;
$listings[$i] = array_filter($listings[$i], function ($element){ 
    return ($element[$i] > $NUM);
}); 

with the following codeblock:

   $num = 2;
   $sentence = $listings[$i];
   //divide the each sentence in words
   $words = explode(',',$sentence);
   $resSentence = [];
   //check each words if their length is more then $num
   foreach($words as $word){
       if(strlen($word) > $num){
           $resSentence[] = $word;
       }
   }
   $listings[$i] = implode(' ', $resSentence);

Update I have check out this program below, is it the whole think what you want?

<?php
$listing_cat = ['Chocolate Manufacturers,C', ',,,Chocolate Manufacturers,,Chocolate Manufacturers,,Chocolate Manufacturers', ''];
$num = 2;
for ($i = 0; $i < count($listing_cat); $i++) {

    $listing_cat[$i] = rtrim(ltrim($listing_cat[$i], ','), ',');
    $sentence = $listing_cat[$i];

    //divide the each sentence in words
    $words = explode(',', $sentence);
    $resSentence = [];

    //check each words if their length is more then $num
    foreach ($words as $word) {
        if (strlen($word) > $num) {
            $resSentence[] = $word;
        }
    }
    $listing_cat[$i] = implode($resSentence);
}
var_dump($listing_cat);