mysql manipulation with php

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Hi i am not much of a techy guy although i know a few things about programming. Please i want to know how i can Update a row with data from another row in the same table in mySQL using PHP from front-end. As in i want to be able to manipulate the data from another row from a html (form) based page using php in the back end to manipulate the data values in the rows. For instance in a row with primary key ID 1 has a column named “Salary” and data value 2000 i want to be able to move 500 from the 2000 to another row with ID 2 under column “Salary” then the balance of Id2’s column salary will be equals to the addition of 500 while 500 will be deducted from Id1’s Salary leaving Id1 with 1500 as balance.