将三个阵列组合成一个内部foreach

$cats_array = array(1,7,28);

foreach ($cats_array as $category) {
    $category_field_query = "SELECT fields 
                             FROM categories 
                             WHERE status = 1 AND id = $category";
    $category_field_query_run = mysqli_query($connect, $category_field_query);
    $cat_field = mysqli_fetch_object($category_field_query_run);

    $field = explode(",", $cat_field->fields); /* Explode ',' from '/'3'/,' */
    $field = str_replace("/","",$field); /* Delete all '/' */

    print_r($field);
}

Inside foreach loop, my query returns something like that /7/,/13/,/24/ from fields for every turn, then I clean them from slashes and commas.

My goal is collect all that arrays inside one array.

I tried to create an empty array outside of foreach and sum all in it but it returned empty.

You have to be define $filed as a array before foreach loop. now you can store value into $filed. note that you have to multidimensional array required to store value.

$cats_array = array(1,7,28);
$field = array();
foreach ($cats_array as $category) {
    $category_field_query = "SELECT fields FROM categories WHERE status = 1 AND id = $category";
    $category_field_query_run = mysqli_query($connect, $category_field_query);
    $cat_field = mysqli_fetch_object($category_field_query_run);

    $field1 = explode(",", $cat_field->fields); /* Explode ',' from '/'3'/,' */
    $field[] = str_replace("/","",$field1); /* Delete all '/' */
} 
print_r($field);

I don't understand why do you store the fields like that, but here is a possible solution:

$fields = array();
$cats_array = array(1,7,28);

foreach ($cats_array as $category) {
    $category_field_query = "SELECT fields FROM categories WHERE status = 1 AND id = $category";
    $category_field_query_run = mysqli_query($connect, $category_field_query);
    $cat_field = mysqli_fetch_object($category_field_query_run);

    preg_match_all('/\/(\d+)\//', $cat_field->fields, $matches);

    if (!empty($matches[1])) {
         $fields = array_merge($fields, $matches[1]);
    }
}

print_r($fields);

$fields = array();
$cats_array = array(1,7,28);
foreach ($cats_array as $category) {
    $category_field_query = "SELECT fields FROM categories WHERE status = 1 AND id= $category";
    $category_field_query_run = mysqli_query($connect, $category_field_query);
    $cat_field = mysqli_fetch_object($category_field_query_run);
    $field = explode(",", $cat_field->fields); // Explode ',' from '/'3'/,'
    $field = str_replace("/", "", $field);
    $fields[] = $field;
}
print_r($fields);