decoct()

The decoct() function converts a decimal (base-10) integer into its octal (base-8) string representation. It is PHP's built-in tool for the decimal-to-octal direction of base conversion, the inverse of octdec().

Octal still shows up in real code — most notably in Unix file permissions, where 0755 and 0644 are octal values. decoct() lets you turn a computed permission integer back into the familiar three-digit string.

Syntax

decoct(int $num): string

• $num — the decimal integer to convert. If a float is passed, it is truncated to an integer first.
• Return value — a string containing the octal representation. The result is a string, not a number, because octal values are typically displayed rather than used in further arithmetic.

The largest number that can be converted depends on the platform's integer size (PHP_INT_MAX). Negative numbers are interpreted using their two's-complement bit pattern for the platform's word size.

Basic example

Decimal 15 is 17 in octal (1 * 8 + 7 = 15). Note that echo prints 17 with no leading zero — decoct() never adds the 0 prefix that octal literals use in source code.

Common values

<?php
echo decoct(8);   // 10
echo "\n";
echo decoct(64);  // 100
echo "\n";
echo decoct(493); // 755  (the rwxr-xr-x permission set)
?>

Because 493 in octal is 755, decoct() is handy when you read a numeric file mode and want to display it the way chmod expects.

Reading back a file's permission bits

fileperms() returns the mode including file-type bits, so mask with & 0777 to keep just the permission portion before converting:

<?php
$mode = 0755;                 // octal literal -> stored as decimal 493
echo decoct($mode & 0777);    // 755
?>

Gotchas

• Output is a string with no leading zero. If you need the conventional four-character form, prepend it yourself: '0' . decoct(493) gives "0755".
• Floats are truncated, not rounded. decoct(15.9) returns "17" (same as decoct(15)), because the float is cast to int first.
• It only handles integers. For converting between arbitrary bases, use base_convert() instead.

Related functions

• octdec() — the inverse: octal string back to decimal.
• dechex() and decbin() — convert decimal to hexadecimal and binary.
• base_convert() — convert a number between any two bases from 2 to 36.

Practice