This document discusses the precision of floating point numbers in PHP. Floating point numbers are used to store fractional numbers, but they are only approximated due to limitations of binary representation. PHP variables can hold either strings or numbers. To use a number as a float, it needs to be cast. The precision directive determines how many digits are displayed for floats converted to strings. It defaults to 14 digits, but microtime() returns timestamps with microseconds requiring 16 digits. Increasing the precision directive or using sprintf() allows retrieving the full precision of floats.

1. Perfectly Problematic PHP Precision
Or “Dude Where's My Digits?”
About the precision of floating point numbers in PHP
By Daniel Rhodes
Of Warp Asylum Ltd
http://www.warpasylum.co.uk

2. Floating what now?
● A floating point number is how we store fractional
numbers in computing
● For example, 1.23, 3.1415, 0.05 all need to be stored
as floating point numbers
● The decimal point “floats” as we can store different
sized whole parts and different accuracies of the
fractional part. For example 34535345345.2 or
2.6745354677

3. OK, so how do we do this in PHP?
● Well, our first little problemette is that PHP is
mostly untyped. The same PHP variable can hold a
string or a number. The variable will be treated as
numbery if it looks like a number, else it will be
treated as stringy.
● This is actually quite useful given that Hyper TEXT
transfer protocol is PHP's biggest context.
● But in our case...

4. Floating point variables
●
$float = (float) 3.1415; //cast from numeric
●
$float = (float) '3.1415'; //cast from string
●
$float = floatval(3.1415); //get float value
●
$float = floatval('3.1415'); //get float value
●
$float = '3.1415'; settype($float, 'float'); //set as
● After any of the above, $float will be internally
represented as an actual floating point number

5. Advanced problems
● OK, that's the basics finished. But there's another,
less obvious problem to be aware of.
● Let's say we want to add metrics recording (of every
button and link clicked for example) to our website,
and we want to save each click event with its
timestamp in our NoSQL database...

6. UNIX timestamp with microseconds
● According to php.net, microtime(true) will return
the current UNIX timestamp – down to microsecond
(ie. 6 digit) accuracy – as a float.
● Let's give it a whirl:
● $ts = microtime(true); echo $ts;
● Gives: 1379523470.4252
● This is not 6 digit accuracy!

7. Dude where's my digits?
● The good news is that internally we do indeed have
6 digits
● The bad news is that, because we used echo, our
float got converted to a stringy format which means
that some dark, mysterious forces come into play.
Namely...
● The precision directive in php.ini [or ini_set()]

8. Peculiar PHP precision
● The little-know precision directive sets the
maximum precision of floats when echoed or stored
as stringy.
● The current default is 14 (2 less than we need for
microsecond timestamps)
● Kicks in when you (or PHP!) echo(), print(), cast
as string or json_encode() a float variable
● Set it in php.ini or with ini_set('precision', 16)

9. Increasing the precision
● OK, let's try our timestamp example again but with
increased precision:
● ini_set('precision', 16); $ts = microtime(true); echo
$ts;
● Gives: 1379525237.5543571
● 6 digits!

10. An interesting exception
● As mentioned, echo() and json_encode() etc will
use the precision directive
● BUT the stringy sprintf() family of functions do not
use the precision directive
● Let's try it:
● $ts = microtime(true); echo sprintf(“%f”, $ts);
● Gives: 1379534265.381266
● Nice!

11. About sprintf()
●
sprintf(“%f”, $x) defaults to 6 decimal digits
● But we can specify the precision with, for example,
sprintf(“%0.8f”, $x) for 8 decimal digits
● So, if appropriate, we can use this instead of
tinkering with the precision directive

12. Summary
● Floating point variables will have precision applied
when made stringy by whatever means
●
precision specifies the total number of digits to
print – before and after the point
●
precision is easily changed in php.ini or with
ini_set()
●
sprintf() [and related] does not apply precision
and is configurable per use

13. Further reading
● http://php.net/manual/en/language.types.type-juggling.php
● http://www.php.net/manual/en/language.types.float.php
● http://floating-point-gui.de (float arithmetic)
● http://www.php.net/manual/en/ini.core.php#ini.serialize-precision
● http://www.leaseweblabs.com/2013/06/the-php-floating-point-precision-is-wrong-by-default
●