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How to Use PHP's bindec() and decbin() Functions to Check if a Specific Binary Bit is 1?

M66 2025-06-23

In PHP programming, when dealing with binary data, it is often necessary to check whether a specific binary bit is 1. PHP provides two very useful functions, bindec() and decbin(), which are used to convert binary to decimal and decimal to binary, respectively. This article will show you how to combine these two functions to check if a specific binary bit is 1.

Introduction to bindec() and decbin()

  • bindec(string $binary_string): int
    Converts a binary string to its corresponding decimal integer.

  • decbin(int $decimal): string
    Converts a decimal integer to its corresponding binary string.

For example:

echo bindec("101");  // Outputs 5
echo decbin(5);      // Outputs "101"

How to Check if a Specific Binary Bit is 1

Suppose we have a decimal number and want to check if the $posth bit (starting from the right and counting from 0) in its binary representation is 1. The approach is as follows:

  1. Convert the number to a binary string.

  2. Calculate the index of the target bit based on the string length.

  3. Check if the character at that position is '1'.

Sample Code

<?php
function isBitSet($decimalNumber, $pos) {
    // Convert decimal to binary string
    $binaryString = decbin($decimalNumber);
$index = strlen($binaryString) - 1 - $pos;

// If the index is less than 0, it means the bit is out of range, return false by default
if ($index < 0) {
    return false;
}

// Check if the corresponding bit is '1'
return $binaryString[$index] === '1';

}

// Test
$number = 13; // Binary is 1101
$position = 2; // Second bit from the right, 0-indexed, the binary is 1
if (isBitSet($number, $position)) {
echo "The {$position}th bit is 1";
} else {
echo "The {$position}th bit is not 1";
}
?>

Output:

The 2nd bit is 1

Additional Notes

  • The bit positions are counted from right to left, with the least significant bit being 0.

  • When using decbin() to convert a decimal number to a binary string, the length of the string may be shorter than the expected number of bits, and bits beyond the length are assumed to be 0.

  • If you want to check multiple bits, you can combine loops or bitwise operations.

Using Bitwise Operations Instead of String Operations (Recommended)

While using decbin() and string indexing can achieve the desired functionality, bitwise operations are more efficient and the code is more concise:

function isBitSetBitwise($number, $pos) {
    return (($number >> $pos) & 1) === 1;
}

You can use the above function to check if a specific bit is 1 as well.

Conclusion

This article introduced how to use PHP's bindec() and decbin() functions together to check if a specific binary bit is 1. This is very helpful for learning and understanding binary bit manipulation. However, for real-world projects, it is recommended to use bitwise operations to improve performance and code readability.

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