How to avoid floating point precision issues when using bindec()

M66 2025-06-02

In PHP, the bindec() function is used to convert binary strings to decimal numbers. Its basic usage is very simple:

 <?php
$binary = "1101";
$decimal = bindec($binary);
echo $decimal; // Output 13
?>

However, when binary strings are very long, especially if they exceed the range that PHP can accurately represent, bindec() may result in errors due to floating-point precision limitations. This is because bindec() returns a value of a floating point number type, while PHP has limited floating point number accuracy, and the longest number of integer digits that can be accurately represented is about 15 digits.

Why are there any accuracy problems?

PHP floating point numbers are based on the IEEE 754 double precision standard, and their accuracy limit means that when the binary string exceeds a certain length, the conversion result will be rounded, resulting in inaccurate values. For example:

 <?php
$longBinary = "111111111111111111111111111111111111"; // 35Bit binary
echo bindec($longBinary); // The results may be inaccurate
?>

In this case, the result returned by bindec() will cause errors due to floating point accuracy limitations.

How to avoid accuracy errors?

Solution 1: Use BCMath to extend manual conversion

PHP's BCMath extension supports arbitrary length numerical operations, which can be used to manually implement binary to decimal to avoid loss of floating point accuracy:

 <?php
function bindec_bcmath(string $binary): string {
    $decimal = '0';
    $len = strlen($binary);
    for ($i = 0; $i < $len; $i++) {
        $decimal = bcmul($decimal, '2');
        if ($binary[$i] === '1') {
            $decimal = bcadd($decimal, '1');
        }
    }
    return $decimal;
}

$longBinary = "111111111111111111111111111111111111";
echo bindec_bcmath($longBinary); // Precise conversion
?>

Solution 2: Use GMP extension

GMP (GNU Multi-precision) extension also supports large-number operations and provides methods for converting directly from binary strings:

 <?php
$longBinary = "111111111111111111111111111111111111";
$decimal = gmp_strval(gmp_init($longBinary, 2), 10);
echo $decimal;
?>

This method is simple and efficient, suitable for handling large numbers.

Solution 3: Custom segmentation conversion (not recommended)

Converting binary strings to decimal by segmenting can also be done, but the implementation is more complex and inefficient. BCMath or GMP is recommended.

Summarize

bindec() is suitable for conversion of short binary strings, exceeding the floating point accuracy limit will lead to errors.
Using BCMath or GMP extensions can avoid accuracy loss and correctly convert large numbers.

bindec