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When working with hex strings, PHP provides a built-in function hexdec() that converts hexadecimal numbers to decimal. However, when using hexdec() , many developers will subconsciously enter a string with the "0x" prefix (such as "0xFF" ), but find that the returned value is not the expected 255, but 0. This situation often makes people confused: it is clearly a legal hexadecimal number, why is the result wrong?
This article will analyze this phenomenon in depth and provide the correct way to use it.
PHP's hexdec() function is designed to handle "pure hex strings", that is, it only accepts characters such as 0–9 and a–f (case insensitive) as input. If the passed string contains content that does not belong to the hexadecimal character set, PHP will read from the beginning of the string until the first illegal character is encountered, and then truncate and convert the previous legitimate part.
<?php
echo hexdec("FF"); // Output: 255
?>
This is a typical example that meets expectations.
Now let’s see the root cause of the problem:
<?php
echo hexdec("0xFF"); // Output: 0
?>
At this time, the output is not 255, but 0. The reason is simple: hexdec() will stop parsing when it encounters the first illegal character, and the first character of the string "0xFF" is 0 , which is legal; the second character is x , which is an illegal hexadecimal character. Therefore, PHP only parses the first character 0 , so it returns 0.
In many programming languages (such as C/C++, JavaScript, Python), the writing method with the "0x" prefix is a legal hexadecimal format. Therefore, some developers will naturally pass "0xFF" to PHP's hexdec() as legal input, without realizing that PHP does not recognize this prefix.
A general and simple solution is to use ltrim() to process the string, removing possible "0x" or "0X" prefixes:
<?php
$input = "0xFF";
$clean = ltrim($input, "0xX"); // Will remove all "0", "x", "X" Starting characters（Inaccurate）
echo hexdec($clean); // Output: 255
?>
But there are also pitfalls in this way of writing, because ltrim("0x0F", "0xX") will get "F" , which may not be what we want. A safer way is to use preg_replace() to remove the prefix exactly:
<?php
$input = "0xFF";
$clean = preg_replace('/^0x/i', '', $input);
echo hexdec($clean); // Output: 255
?>
This will only remove the beginning "0x" or "0X" without destroying the number itself.
If you encounter hexadecimal strings when processing parameters in the URL, especially from addresses like https://gitbox.net/api?color=0xFF00FF , special attention should be paid to:
<?php
$hex = $_GET['color']; // For example: 0xFF00FF
$hex = preg_replace('/^0x/i', '', $hex);
$decimal = hexdec($hex);
echo $decimal; // Output: 16711935
?>
This ensures that you can correctly parse hexadecimal numbers with the "0x" prefix.
PHP's hexdec() does not recognize the "0x" prefix, it only treats the string as a collection of pure hexadecimal characters. Therefore, before using hexdec() , developers need to manually clean the string and remove the "0x" prefix to avoid getting wrong values. Understanding and handling this detail is the key to ensuring the accuracy of PHP programs.
