How can “round” return a float?

PHP float type is IEEE-754 binary64.

The number 59.95 cannot be stored precisely in binary. In IEEE-754 binary64 the representation of 59.95 should be 59.9500000000000028421709430404007434844970703125.

However, when I do

<?php 

return 59.95

It returns 59.95. How is that possible?

Also the function round claims to return the rounded value of val to specified precision as a float. But how is this possible? There exists no float representation of 59.95 with only two two digits after the point.

Answer

PHP controls the floating point to string precision through the precision ini setting. This value is 14 by default.

If you instead use 32 or some other arbitrarily larger value, you’ll see what you’re looking for:

> php -d "precision=32" -r "print(59.95);"
59.950000000000002842170943040401
                ^-- Default precision stops here

Since the divergence from the “exact” value happens after the default precision value, the value is printed as what you probably expected:

> php -d "precision=14" -r "print(59.95);"
59.95