decbin()

The decbin() function converts a decimal (base‑10) integer into its binary (base‑2) representation and returns it as a string. It is the counterpart of bindec(), which converts the other way. This page covers the syntax, the return value, the gotchas with negative numbers, and how to format the output.

Syntax

decbin(int $num): string

• $num — the decimal integer to convert. Floats are silently truncated to integers, and booleans are cast (true becomes 1).
• Return value — a string of 0 and 1 characters. Note it is a string, not a number, so leading zeros are never produced and the result is safe to concatenate or pad.

Basic Example

We assign a decimal value, pass it to decbin(), and print the returned string. Decimal 10 is 1010 in binary (8 + 2).

Converting Several Values

Because the function returns a plain string, you can format it inline. Here are a few common conversions:

<?php
echo decbin(0);   // 0
echo "\n";
echo decbin(1);   // 1
echo "\n";
echo decbin(255); // 11111111
echo "\n";
echo decbin(256); // 100000000
?>

255 is the largest value that fits in one byte, which is why it maps to eight 1 bits.

Padding the Output to a Fixed Width

decbin() never adds leading zeros, so decbin(5) returns "101", not "00000101". When you want a fixed bit width (for example, to display every value as a byte), wrap it in str_pad():

<?php
$value = 5;

// Pad to 8 bits with leading zeros
echo str_pad(decbin($value), 8, "0", STR_PAD_LEFT); // 00000101
?>

This works precisely because the return value is a string.

Negative Numbers

PHP has no dedicated sign bit handling here: a negative integer is interpreted as an unsigned value using two's‑complement across the platform's integer width. On a 64‑bit build, decbin(-1) returns 64 ones:

<?php
echo decbin(-1);
// 1111111111111111111111111111111111111111111111111111111111111111
?>

If you need a fixed, predictable width, mask the value first — for example decbin(-1 & 0xFF) yields 11111111 (an 8‑bit view).

Round‑Tripping with bindec()

decbin() and bindec() are inverses, so converting a number to binary and back returns the original value:

<?php
$n = 42;
$binary = decbin($n);     // "101010"
$back = bindec($binary);  // 42

echo $back; // 42
?>

When Would I Use It?

• Inspecting or displaying bit patterns while debugging bitwise operations (&, |, ^, <<, >>).
• Building binary string representations for teaching, logging, or simple data encoding.
• Working with permission/flag bitmasks where seeing the individual bits is helpful.

Related Functions

PHP ships a family of base‑conversion helpers. Pick the one that matches your source and target base:

• bindec() — binary string to decimal.
• dechex() — decimal to hexadecimal.
• hexdec() — hexadecimal to decimal.
• decoct() — decimal to octal.
• octdec() — octal to decimal.
• base_convert() — convert between arbitrary bases (2–36).

Conclusion

The decbin() function turns a decimal integer into a binary string. Remember three things: the result is a string (so use str_pad() for fixed widths), negative numbers are shown in two's‑complement across the full integer width, and bindec() reverses the conversion.

Practice