SELECT *,@k as k
FROM vpdm.main_arh 
WHERE fullpath NOT LIKE \"%Архив аннулированных%\" $ids  $nids
AND 
CASE
WHEN info LIKE \"%$string%.pdf\" AND ids=\"Тех_Усл\" THEN @k:=1
[….skip….]
WHEN info LIKE \"%$st3%.pdf\" THEN @k:=9
ELSE @k=10
END
Order by k
LIMIT 10;

SELECT *,@k as k
FROM vpdm.main_arh 
WHERE fullpath NOT LIKE "%Архив аннулированных%"   
AND 
CASE
WHEN info LIKE "%У38_424_005%01_02%.pdf" AND ids="Тех_Усл" THEN @k:=1
WHEN info LIKE "У38_424_005%01_02%.pdf" AND ids="npk5_pdf_arh" THEN @k:=2
WHEN info LIKE "У38_424_005%01_02%.pdf" AND ids="СМК_ОАО_Морион" THEN @k:=3
WHEN info LIKE "У38_424_005%01_02%.pdf" THEN @k:=4
WHEN info LIKE "%У38_424_005%01_02%.pdf" THEN @k:=5
WHEN decnum LIKE "%У38_424_005%01_02%" and info LIKE "%.pdf" THEN @k:=6
WHEN info LIKE "%У38_424_005%01_02%.pdf" THEN @k:=7
WHEN info LIKE "%У38_424_005%.pdf" THEN @k:=8
WHEN info LIKE "%У38_424_005%01_02%.pdf" THEN @k:=9
ELSE @k:=10
END
Order by k
LIMIT 10

//---- Результат:
Array
(
    [0] => Array
        (
            [id] => 1
            [decnum] => 
            [fullpath] => c:/VirtHoshs/PdfArh/Новое/Горелка двухщелевая для резки колб.pdf
            [info] => Горелка двухщелевая для резки колб.pdf
            [ids] => npk5_pdf_arh
            [k] =>       //<--- Почему-то пустое значение 
        )

    [1] => Array
        (
            [id] => 2
            [decnum] => 
            [fullpath] => c:/VirtHoshs/PdfArh/Новое/Держатель к двухщелевой горелке.pdf
            [info] => Держатель к двухщелевой горелке.pdf
            [ids] => npk5_pdf_arh
            [k] =>       //<--- Почему-то пустое значение 
        )