我的foreach循环中的错误是什么？

Following is my multidimensional array named $prev_map_data:

Array
(
    [0] => Array
        (
            [class_id] => 2
            [class_name] => II
            [class_checked] => 1
            [class_subjects] => Array
                (
                    [0] => Array
                        (
                            [cs_map_id] => 81
                            [subject_name] => 11 Engllish
                            [subject_checked] => 1
                            [teacher_cs_id] => 81
                        )

                )

        )
[1] => Array
    (
        [class_id] => 3
        [class_name] => III
        [class_checked] => 1
        [class_subjects] => Array
            (
                [0] => Array
                    (
                        [cs_map_id] => 155
                        [subject_name] => Hidi
                        [subject_checked] => 1
                        [teacher_cs_id] => 155
                    )

                [1] => Array
                    (
                        [cs_map_id] => 156
                        [subject_name] => 11 Maths
                        [subject_checked] => 1
                        [teacher_cs_id] => 156
                    )

                [2] => Array
                    (
                        [cs_map_id] => 157
                        [subject_name] => 11 Science
                        [subject_checked] => 1
                        [teacher_cs_id] => 157
                    )

            )

    )

)

I want to get all the values from key [class_id] one by one and push the values in an array $data. But getting the warning

Warning: Invalid argument supplied for foreach()

I'm not understanding what is wrong with my code. Can any one help me to improve my code and append the values to the new array? Thanks in advance. My code is as follows :

 $prev_map_data  = $objTeacherClassesSubjects->GetClassSubjectMappingsbyTeacherId ($request, $teacher_class_subjects_error_messages);
   foreach($prev_map_data as $map_id) {
              $cls_data[] = $map_id['class_id'];
          }
    }

Try this code

$teacher_class_subjects_error_messages);
   foreach($prev_map_data as $iKey => $iVal) {
              $cls_data[] = $iVal['class_id'];
          }
    }

$prev_map_data is not an array. use var_export($prev_map_data); to see if it is an array

if(!empty($prev_map_data) && is_array($prev_map_data)) {
  foreach($prev_map_data as $key=>$map_id) {
     if(is_array($map_id))
       $cls_data[] = $map_id['class_id'];
     else
       echo '$map_id is not an array!';
  }
} else {
  echo '$prev_map_data is not an array or it is empty!';
}