Description:
------------
Running UPDATE...SET...CASE query in console works, but not from php code -> sets to NULL;


Reproduce code:
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UPDATE...
SET...
products.products_price = @nPrice:= (CASE LENGTH(ROUND(@price:= @finalprice * @tax:= ((tax_rates.tax_rate/100)+1)))

 WHEN 10 THEN INSERT(ROUND(@price),8,3,'900')/@tax
 WHEN 9 THEN INSERT(ROUND(@price),7,3,'900')/@tax
 WHEN 8 THEN INSERT(ROUND(@price),6,3,'900')/@tax
 WHEN 7 THEN INSERT(ROUND(@price),5,3,'900')/@tax
 WHEN 6 THEN INSERT(ROUND(@price),4,3,'900')/@tax
 WHEN 5 THEN INSERT(ROUND(@price),3,3,'900')/@tax
 WHEN 4 THEN INSERT(ROUND(@price),3,2,'99')/@tax
 WHEN 3 THEN INSERT(ROUND(@price),2,2,'99')/@tax
 WHEN 2 THEN INSERT(ROUND(@price),2,1,'9')/@tax
 WHEN 1 THEN ROUND(@price)/@tax ELSE @price END)
 * 1,
 products.products_price = @nPrice,

Expected result:
----------------
From console; the code works, @nPrice is inserted successfully.

From php code; only the CASE statement is not processed!
There is no MySQL errors.

Actual result:
--------------
@nPrice is set to NULL.