what mistake i did in mysql query
Use Backtick Sign  around columns names. As you have a column desc which is a keyword in MySQL, So you need to put Backtick around column the name

insert into hall_booking
(`name`,`address`,`event_type`,`hallNo`,`arrival_time`,
`arrival_date`,`dep_time`,`dep_date`,`identity`,`emailid`,
`contact`,`total_members`,`desc`,`catering`,`service`,
`decoration`,`other`) 
values
('$nm','$add','$typ','$roomno','$arrv',
'$arrivaldate','$departure','$dDate','$idt','$emailid',
'$cont','$desc','200','200','200',
'200')


And one more thing in your insert you are missing value for one column, as you have 17 columns to insert but you are inserting only 16 values.

I hope you are missing value for column total_members,

Your insert should be like below

insert into hall_booking
(`name`,`address`,`event_type`,`hallNo`,`arrival_time`,
`arrival_date`,`dep_time`,`dep_date`,`identity`,`emailid`,
`contact`,`total_members`,`desc`,`catering`,`service`,
`decoration`,`other`) 
values
('$nm','$add','$typ','$roomno','$arrv',
'$arrivaldate','$departure','$dDate','$idt','$emailid',
'$cont','$total_members','$desc','200','200','200', /** Added total_members **/
'200')