mysql_query('UPDATE myTable SET say="$newSay" WHERE id=100');
But the SQL above doesn’t work because the quotation mark in where clause of the SQL seems, I think, to collide with the quotation marks which is inside the value.
Incidentally, if these are actually imperial measurements in feet and inches, I’d recommend you store them in inches and just convert them for display.
If you’re not doing this for a programmatical reason (Read: You need the value elsewhere in your PHP code), instead do the replacement using the MySQL engine itself:
UPDATE myTable SET say=REPLACE(REPLACE(say,"A","AA"),"B","BB") WHERE id=100;