is_double()

Introduction

is_double() was an alias of is_float() that checks whether a variable holds a floating-point number. A float (also called a double) is the data type PHP uses for numbers with a fractional part, such as 3.14 or 1.0e6.

Important: is_double() was deprecated long ago and removed in PHP 8.0.0. In any PHP version, prefer is_float() — it is identical in behavior, supported, and clearer to read. This page documents is_double() so you can recognize and migrate legacy code that still uses it.

This chapter covers the syntax, what counts as a float, how the return value behaves, the recommended replacement, and the common gotchas.

Syntax

is_double(mixed $value): bool

It takes one parameter, $value, the variable to test, and returns true if $value is of type float, false otherwise. The check is on the type, not on whether the number looks decimal — see the gotchas below.

What counts as a float

is_double() (and is_float()) returns true only when the variable's runtime type is float. Numeric strings and whole-number integers are not floats:

<?php
var_dump(is_float(3.14));    // bool(true)
var_dump(is_float(1.0e6));   // bool(true)  — scientific notation is a float
var_dump(is_float(42));      // bool(false) — this is an int
var_dump(is_float("3.14"));  // bool(false) — this is a string
var_dump(is_float(10 / 2));  // bool(true)  — division always yields a float in PHP
?>

Note that 10 / 2 is true: in PHP, the / operator always produces a float, even when the result is mathematically a whole number.

Example Usage

Because echoing booleans is misleading (true prints 1, false prints nothing), use var_dump() to see the real result:

$price is a genuine float, so the check is true. $count is an integer and $label is a string, so both are false.

The modern replacement: is_float()

Replacing is_double() is a straight rename — the arguments and return value are the same:

<?php
// Legacy (removed in PHP 8.0)
// if (is_double($value)) { ... }

// Modern, supported everywhere
$value = 9.99;
if (is_float($value)) {
    echo "It's a float";
}
?>

If you need to accept numbers that might be a float, an int, or a numeric string (for example, form input), reach for is_numeric() instead, or convert with floatval().

Common gotchas

• Numeric strings fail the check. is_float("3.14") is false. User input from forms or $_GET arrives as strings; validate with is_numeric() first, then cast with floatval().
• Don't compare floats for exact equality. Because of binary rounding, 0.1 + 0.2 == 0.3 is false. Compare within a small tolerance instead.
• is_double is dead in PHP 8+. Calling it on PHP 8.0 or later raises a fatal Error: Call to undefined function is_double().

See also

• is_float() — the current, supported function.
• is_int() — test for integers.
• is_numeric() — test for numbers or numeric strings.
• floatval() — convert a value to a float.
• gettype() — inspect a variable's type directly.

Conclusion

is_double() is a removed alias of is_float(). For new code, always use is_float() to test whether a value is a floating-point number, and only keep is_double() in mind so you can update legacy projects when you migrate them to PHP 8 or later.

Practice