Comparing Alphanumeric values in a range

Hi, I have a task to generate a new document number for a given range. This document number is alphanumeric. Here is the problem: $lowrange1 = "900000"; $highrange1 = "HZZZZZ"; $currentno = "90CZZZ"; I expect that the next number in the series is: $nextnum = "90D000"; Before using this next number in the series, I need to confirm that the number falls within the given range. So I do something like this: if ((strval($nextnum) >= strval($lowrange1)) && (strval($nextnum) <= strval($highrange1))) { // we have a valid number in the range } else { // not in the range } Now.. the problem is - PHP thinks that 90D000 is less than $lowrange1. I individually tested them and $lowrange1 was consistently greater than the number. I put the strval function around each variable to ensure it was comparing strings and not numbers for those variables that only contain numbers. This seems to be a problem with the letter "D" and "E" for all positions within the 6 character alphanumeric number, except for the last position. ie. 900000D works - $lowrange1 is less than this number and $highrange1 is greater than this number. For all other positions where the letter "D" and "E" are generated: ie. 9000D0, 900D00, 90D000, 9D0000 and 9000E0, 900E00, 90E000, 9E0000 Is there a known problem with this? Why it is only happening with the letters "D" and "E"? If I have 900F00 - this works, and so on for the rest of the letters of the alphabet. Thanks, Sam