hexdec()

The PHP hexdec() function converts a hexadecimal (base-16) string into its decimal (base-10) equivalent. It is the inverse of dechex(), and it is handy whenever you read hex values that come from colors, byte dumps, hashes, or low-level protocols and need to do arithmetic on them.

This page covers the syntax, what the return type really is, how hexdec() handles invalid characters, the large-number gotcha, and how it relates to the other base-conversion functions.

Syntax

hexdec(string $hex_string): int|float

• $hex_string — the hexadecimal string to convert. Hex digits are 0-9 and a-f (case-insensitive, so FF and ff are equal).
• Return value — the decimal value as an int, or a float when the number is too large to fit in a PHP integer.

A Basic Example

1a in base-16 means 1 × 16 + 10 = 26, which is the value printed. The input is a string here — that is the normal and recommended way to pass hex values, because a literal like 1a is not valid PHP and 0x1a would already be an integer.

Converting Several Values

hexdec() is case-insensitive and ignores any leading 0x prefix that you might copy from another tool:

<?php
echo hexdec("ff"), "\n";   // 255
echo hexdec("FF"), "\n";   // 255 (same — case does not matter)
echo hexdec("100"), "\n";  // 256
echo hexdec("7fff"), "\n"; // 32767
echo hexdec("0x1A"), "\n"; // 26  (the "0x" is ignored)
?>

A common real-world use is splitting a CSS hex color into its red, green, and blue channels:

<?php
$color = "ff8800";

$red   = hexdec(substr($color, 0, 2)); // 255
$green = hexdec(substr($color, 2, 2)); // 136
$blue  = hexdec(substr($color, 4, 2)); // 0

echo "rgb($red, $green, $blue)"; // rgb(255, 136, 0)
?>

How Invalid Characters Are Handled

hexdec() does not throw an error on a malformed string. Instead, every character that is not a valid hex digit is silently ignored (and, since PHP 7.4, a deprecation/E_WARNING notice is emitted). The remaining valid digits are still converted:

<?php
echo hexdec("a0.5"), "\n"; // 2565 — the "." is dropped, so "a05" is converted
echo hexdec("xyz1f"), "\n"; // 31  — only "1f" is valid hex
?>

Because the failure is silent, validate input yourself when it might be untrusted. ctype_xdigit() checks that a string contains only hex digits:

<?php
$input = "a0.5";

if (ctype_xdigit($input)) {
    echo hexdec($input);
} else {
    echo "Not a valid hexadecimal string";
}
// Not a valid hexadecimal string
?>

Large Numbers Return a Float

PHP integers are platform-bound (64-bit on most modern systems). When a hex value exceeds PHP_INT_MAX, hexdec() returns a float instead of an int so the value is not truncated — though very large floats lose precision:

<?php
var_dump(hexdec("1a"));               // int(26)
var_dump(hexdec("ffffffffffffffff")); // float(1.8446744073709552E+19)
?>

If you need exact arithmetic on huge hex numbers, reach for the BCMath or GMP extensions instead.

Related Base-Conversion Functions

hexdec() is one of a family of base converters. Pick the one that matches your input and output bases:

For arbitrary bases (anything from base-2 to base-36) use base_convert(), and to turn a hex string into raw bytes use hex2bin().

Conclusion

hexdec() turns a hexadecimal string into a decimal number, returning an int (or a float for very large values). Remember that it ignores invalid characters silently — so validate untrusted input with ctype_xdigit() first — and use its counterpart dechex() to convert back.

Practice