Restrict of overlap of schedule PHP

I have a PHP code and it work my only problem is if i create a schedule that overlaps the previous schedule

for example i schedule

**7:00 am to 8:30 am ** Succesfully Added

**6:30am to 7:30am ** Error Message: The schedule is conflict with other schedules

**6:30 am to 9:00 am ** Successfully Added (this one overlap a schedule so this needs to be a error)

**7:30am to 9:30am ** Error Message: The schedule is conflict with other schedules

This is the sample of my code

    $d_start = strtotime($datetime_start);
            $d_end = strtotime($datetime_end);
            $roomID = $assembly_hall_id;

            $chk = $this->conn->query("SELECT * FROM `schedule_list` where (('{$d_start}'   
            Between unix_timestamp(datetime_start) and unix_timestamp(datetime_end)) or ('{$d_end}' 
            Between unix_timestamp(datetime_start) and unix_timestamp(datetime_end))) ".(($roomID > 0) ? "
            and assembly_hall_id ='{$roomID}' and sched_status = '1' " : ""))->num_rows;
            
            if($chk > 0 ){
                $resp['status'] = 'failed';
                $resp['err_msg'] = "The schedule is conflict with other schedules.";
            }elseif(strtotime($datetime_start) == null)
            {   
                $resp['status'] = 'failed';
                $resp['err_msg'] = "Date and Time Schedule is Invalid.";
            }

>Solution :

You have not covered the case where d_start<datetime_start AND d_end>datetime_end

Change the SELECT to include this:

...
('{$d_start}' Between unix_timestamp(datetime_start) and unix_timestamp(datetime_end))
 OR
 ('{$d_end}' Between unix_timestamp(datetime_start) and unix_timestamp(datetime_end))
 OR
 (
  '{$d_start}' < unix_timestamp(datetime_start)
  AND
  '{$d_end}' > unix_timestamp(datetime_end)
 )
...

EDIT:
A simpler structure is this:

...
('{$d_start}' < unix_timestamp(datetime_end)
 AND  
 '{$d_end}' > unix_timestamp(datetime_start)) 
...