Description:
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This is the same problem as stated in reports 20365 and 29822. This is indeed not bogus, PHP, C and Javascript are NOT producing the mathematical correct result. Per definition modulus arithmetics create a ring structure containing the values [0,1,...,(n-1)]. THERE ARE NO NEGATIVE NUMBERS! The current behavior is not intuitive and basically incorrect. An integer typecast is not the same as floor.

This is a big problem when shifting table fields (e.g. weekdays). Take a look at spreadsheet software.

//This is the way % should behave
function math_mod($a,$n){$n=abs($n);if($n===0){return NULL;}return $a-$n*floor($a/$n);}

//this function emulates ($a%$n)
function php_mod($a,$n){return $a-$n*(int)($a/$n);} 

Reproduce code:
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echo -5%7 , ' ', fmod(-5,7),' ', 1%0,' ', 3%-2;


Expected result:
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2 2 NULL 1

Actual result:
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-5 -5 NULL 1