Filtering data according to field name and value

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PHP

MySQL

Hi all,
I have a form with dropdownlist containing fieldnames like firstname, lastname
I have a textbox to type the keyword.
According to the fieldname selected in dropdown and the keyword typed in textbox , I have to filter the user details.
I have written a stored procedure with prepared statement. But its not working.

create procedure prcUsers
(
	PvchFieldName varchar(30),
	PvchFieldValue varchar(30)
)
begin
	declare f,v varchar(30);
    set @fname=PvchFieldName;
	set @val=PvchFieldValue;
	set @q= concat('select * from tbmstuser where ',f,'=? like',v,'=?%');
	prepare stmt from @q;	
	execute stmt using @fname, @val;
	deallocate prepare stmt;
end

calling the procedure gives me the following error

Error Code: 1064. You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'NULL' at line 1	0.000 sec

Have to call like this

call prcUsers('vchFirstName','a');