I was reading that you have to be careful with floating point comparisons and calculations in PHP. I’m not an expert at PHP so I thought I would ask the community what is the best way to do accurate decimal calculations in PHP.
I’m developing a timecard application where it calculates all the time entered for the week. Anything over 40 hours is considered overtime. I then want to distribute that overtime back across each time entry based on the percentage that entry is of the total time.
Example:
Monday = 10 hours
Total Time = 50 hours
Total Overtime = 10 hours
Percent of Total time (0.2) = Monday (10 hours) / Total Time (50 hours)
Overtime to Allocate to Monday (2 hours) = Total Overtime (10 hours) * Percent of Total Time (0.2)
Now make a new entry for Monday with 2 hours of overtime and subtract 2 hours from original entry.
New Entries:
Monday (8 hours) = Monday – Overtime to Allocate to Monday (2 hours)
Monday Overtime = 2 hours
Things don’t seem to bad when using integers as your original entries but what will happen when I start using floats and have multiple time entries per day? Each time entry will have part of the total overtime allocated to it but I need to make sure not to allocate more overtime then is available and I also need to make sure the total time for each day and for the whole timecard is the same as originally entered. I read comparing floats in php are not safe. How should I do the calculation. Also, the final entered numbers have to be rounded to 2 decimal points (.##)
Read The Floating-Point Guide to understand why exactly you have to be careful with mixing floating-point numbers and the concept of “decimals” (which has absolutely nothing to do with PHP specifically).
It also has a cheat-sheet for PHP.