How to emulate single precision float operations in PHP? - php

Question

I need to port a simple C program to PHP. Currently we have to start the process and parse it's output. The program is very trivial but it is important for the algorithm to use float as the errors will sum up and the result will be way off.
C example:
#include <stdio.h>
int main( void ) {
printf("%f\n", 123456 * (float)0.99524);
printf("%f\n", 123456 * (double)0.99524);
return 0;
}
PHP example:
<?php
printf("%f\n", 123456 * 0.99524);
?>
The C example will result in 122868.343750 and 122868.349440 while PHP will end up with 122868.349440.
How do I get the C float result in PHP?

Answer 1

There is no way you can do this using built in php functions.
The one using "double" gives you the real result, 100% precise. The float one is wrong.
In PHP float and double are the same type, which is double.
If you need high precision results, that always give the same results, try using BC Math module: http://php.net/bcmath
Example code using BC Math:
$result = bcmul("123456", "0.99524", 6); // gives 122868.34944
$result = number_format($result, 6, ".", ""); // 122868.349440 - appending zeros
echo $result;
Output:
122868.349440
If you really, really want the same result as in the C program, then you have 2 options:
Create your own c-like function by writing a php extension: http://www.google.com/search?q=writing+php+extensions
Talk to your C-program from PHP via function proc_open():
http://www.php.net/manual/en/function.proc-open.php (see also popen(), exec() or shell_exec())

Answer 2

You could always create a PHP module.
Here are a list of resources that I've compiled over time...
http://www.delicious.com/homer6/php+extension
Also, I'd highly recommend reading Sara Goleman's book:
http://blog.simonholywell.com/post/1156691738/15-excellent-resources-for-php-extension-development
Hope that helps...

Answer 3

Floating point numbers have limited precision. Although it depends on
the system, PHP typically uses the IEEE 754 double precision format,
which will give a maximum relative error due to rounding in the order
of 1.11e-16. Non elementary arithmetic operations may give larger
errors, and, of course, error progragation must be considered when
several operations are compounded.
Additionally, rational numbers that are exactly representable as
floating point numbers in base 10, like 0.1 or 0.7, do not have an
exact representation as floating point numbers in base 2, which is
used internally, no matter the size of the mantissa. Hence, they
cannot be converted into their internal binary counterparts without a
small loss of precision. This can lead to confusing results: for
example, floor((0.1+0.7)*10) will usually return 7 instead of the
expected 8, since the internal representation will be something like
7.9999999999999991118....
So never trust floating number results to the last digit, and never
compare floating point numbers for equality. If higher precision is
necessary, the arbitrary precision math functions and gmp functions
are available.
Quoted from : http://php.net/manual/en/language.types.float.php
To change the precision level of PHP , change the precision settings in php.ini