Floating point numbers

Floating point numbers (also known as "floats", "doubles", or "real numbers") can be specified using any of the following syntaxes:

<?php
$a = 1.234;
$b = 1.2e3;
$c = 7E-10;
$d = 1_234.567; // as of PHP 7.4.0
?>

Formally as of PHP 7.4.0 (previously, underscores have not been allowed):

LNUM          [0-9]+(_[0-9]+)*
DNUM          ({LNUM}?"."{LNUM}) | ({LNUM}"."{LNUM}?)
EXPONENT_DNUM (({LNUM} | {DNUM}) [eE][+-]? {LNUM})

The size of a float is platform-dependent, although a maximum of approximately 1.8e308 with a precision of roughly 14 decimal digits is a common value (the 64 bit IEEE format).

Waarschuwing: > ### Floating point precision

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 propagation must be considered when several operations are compounded.

Additionally, rational numbers that are exactly representable as floating point numbers in base 10, like or , 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, will usually return instead of the expected , since the internal representation will be something like . 0.1``0.7``floor((0.1+0.7)*10)``7``8``7.9999999999999991118...

So never trust floating number results to the last digit, and do not compare floating point numbers directly for equality. If higher precision is necessary, the and functions are available. arbitrary precision math functionsgmp

For a "simple" explanation, see the that's also titled "Why don’t my numbers add up?" floating point guide

Converting to float

From strings

If the string is

or leading numeric then it will resolve to the
corresponding float value, otherwise it is converted to zero
().

numeric0

From other types

For values of other types, the conversion is performed by converting the
value to  first and then to . See

for more information.

int``floatConverting to integer

Opmerking: > As certain types have undefined behavior when converting to , this is also the case when converting to . int``float

Comparing floats

As noted in the warning above, testing floating point values for equality is problematic, due to the way that they are represented internally. However, there are ways to make comparisons of floating point values that work around these limitations.

To test floating point values for equality, an upper bound on the relative error due to rounding is used. This value is known as the machine epsilon, or unit roundoff, and is the smallest acceptable difference in calculations.

and  are equal to 5 digits of

precision. $a``$b

Voorbeeld: Comparing Floats

<?php
$a = 1.23456789;
$b = 1.23456780;
$epsilon = 0.00001;

if (abs($a - $b) < $epsilon) {
    echo "true";
}
?>

NaN

Some numeric operations can result in a value represented by the constant . This result represents an undefined or unrepresentable value in floating-point calculations. Any loose or strict comparisons of this value against any other value, including itself, but except true, will have a result of false. NAN

Because represents any number of different values, should not be compared to other values, including itself, and instead should be checked for using . NAN``NAN``is_nan

Vorige ← Enumerations Volgende Integers →

Floating point numbers