Ex: /add <number 1> <number 2> If number < 10 if will doesn't work... If number > 16 if will doesn't work neither...
You'll have to be a bit more descriptive, but I believe you can accomplish this by learning "if statements" in PHP first. Use the internet for that, this isn't the place for general PHP questions.
1. please show proof of a previous attempt 2. please learn how to program on your own before asking a forum about PocketMine API about generic things 3. This needs moved to facepalm...
Better to use if_int to check if its an integer, as you probably dont want to be working with complex floats (decimals).
If you mean only using is_int, this is probably wrong. Spoiler is_int only checks the variable type to be an int. It does not check for a numeric string to represent integer, nor float numbers to represent an integer.
If you cast it to an int, it must be an int, so it is pointless to check anymore. Use ctype_digit() to make sure all numbers are numeric if you don't want to allow decimal or negative. You may also ternarily check the first digit to be a negative sign, and conditionally substr(, 1) it.