Question

   $query="INSERT INTO `main_table` (`count`) VALUES ('$count') WHERE userid='$someid'";

basically I like to insert a new value of count into the variable count in the main_table where the userid equal to a variable someid.

SQL complains the syntax on this statement.

Answer 1

To update an existing row you should use an UPDATE statement instead of an INSERT statement.

UPDATE `main_table`
SET `count` = '$count'
WHERE userid='$someid'

See it working online: sqlfiddle

INSERT is only used for inserting completely new rows into your table. It is not legal to have a WHERE clause on an INSERT statement.