How to update multiple columns in mysql using php

Try like this :

$sql = "UPDATE product_list SET product_name='".$product_name."',product_category='".$product_category."',product_price='".$product_price."',product_description='".$product_description."',size_category='".$size_category."' WHERE product_id=".$product_id;

Reference : https://dev.mysql.com/doc/refman/5.0/en/update.html

You are mixing up query syntax between INSERT and UPDATE queries, the UPDATE syntax is ;

UPDATE TABLE SET col1 = val1, col2=val2... WHERE col1 = val

You shall use the UPDATE query as follows :

$sql = "UPDATE product_list SET product_name = '$product_name', 
product_category = '$product_category' WHERE product_id = $product_id";

Update SQL query, see following method:

Update database_tablename SET column_name1 = column_value1 , column_name2 = column_value2

$sql = "UPDATE product_list SET product_name='".$product_name."',product_category='".$product_category."',product_price='".$product_price."',product_description='".$product_description."',size_category='".$size_category."'";

 $sql = "UPDATE `product_list` SET 
       `product_name` = '$product_name', 
       `product_category` = '$product_category', 
       `product_price` = '$product_price', 
       `product_description` = '$product_description', 
       `product_size_category` = '$size_category' 
  where clause..... (if required) ";