Replacing mysql values

So I am trying to update a table from a MYSQL database with values from another table. I want to replace the values for each row of Columns A and B in table 1 with the values for each row of Columns C and D in table 2.

How do I do this?

 

Is there a condition on how to replace the values? I mean which values from which row in table 1 replaces rows in table 2?

 

Values from Column A that are the same as values from Column C.

 

$query= mysql_query("Select * From table2");
while($row = mysql_fetch_array($query))
{
   $c = $row['C'];
   $d = $row['D'];
   $res= mysql_query("UPDATE table1 SET B='$d' WHERE C='$c'");
}

PHP: