Question

In PHP, I know we shouldn’t do math on floats without things like bcmath, but is the mere act of casting a string to float destructive?

Will expressions like (float)'5.111' == '5.111', always be true? Or will the cast itself change that to something like 5.1110000000000199837 as the number is converted?

The main reason is, just as I use (int) to escape integer values going into a database, I would like to use (float) in the same way, without having to rely on quotes and my escape function.

Answer 1

NO, Casting to a float is almost always destructive.

In your example, 5.111 represented in binary is:

101.00011100011010100111111011111001110110110010001011010000111001...

A float would store 23 digits:

101.0001110001101010011 
(5.1109981536865234375)

A double would store 52 digits:

101.0001110001101010011111101111100111011011001000101
(5.1109999999999988773424774990417063236236572265625)

In this case, there wouldn’t be a difference. However, in larger numbers, it can affect what you display.

For example:

1025.4995

double:

10000000001.011111111101111100111011011001000101101
(1025.499499999999898136593401432037353515625)

float:

10000000001.011111111101
(1025.499267578125)

You can see the precision starts to drop off dramatically after around 8 digits.

The double would round to 1025.4995 whereas the float would be 1025.4993