When I do any php function that gets a size of something, does it measure the size in bytes? bits?

and the size ratio is like this, correct?:
1 kilobyte (KB) 1,024 bytes
1 megabyte (MB) 1,048,576 bytes
1 gigabyte (GB) 1,073,741,824 bytes
1 terabyte (TB) 1,099,511,627,776 bytes
1 petabyte (PB) 1,125,899,906,842,624 bytes

can also be seen as this:
Bit: Smallest unit of measurement.
Byte: 8 bits
Kilobyte: (KB) 1024 bytes.
MegaByte: (MB) 1024 kilobytes (KB)
GigaByte: (GB) 1024 megabytes (MB)

but is the size really important? Could you round it down and say 1kb = 1000 bytes? Does PHP have a built in function to convert these values, or do I have to write all that stuff?