Print non ASCII characters as their hex in PHP

Question

How do I print a string in PHP in which all the non-ASCII characters gets converted to their HEX value (eg. 0x02) and displayed? I want users to know that they are entering non-ASCII values. I don't want to strip them. Instead, I would like to display it so that they can edit and correct the mistakes.

I want to allow users to enter standard tab, new lines, etc (maybe upto ASCII 127).

I tried quoted_printable_encode() but it displays = as =3D. Other non-ASCII characters as =[HEXVAL]. The equal sign creates confusion.

I tried preg_replace('/[[:^print:]]/', '', $string) but it ended up removing tabs, new lines, etc.

Answer 1

This is hard to acheive when it comes to unicode characters. Even valid unicode characters (there are a ton of it) might not being printable because the current font contains no letter definitions for that character. Meaning a German unicode font might not contain all valid Chinese characters for example.

If you just care about ascii, you can use ctype_print() to check if a character is printable or not.

Example:

// test string contains printable and non printable characters
$string = "\x12\x12hello\x10world\x03";

$allowed = array("\x10", /* , ... */);

// iterate through string
for($i=0; $i < strlen($string); $i++) {

    // check if current char is printable
    if(ctype_print($string[$i]) || in_array($string[$i], $allowed)) {
        print $string[$i];
    } else {
        // use printf and ord to print the hex value if
        // it is a non printable character
        printf("\\x%02X", ord($string[$i]));
    }   
}

Output:

\x12\x12hello
world\x03

Answer 2

substr(json_encode((string)$string), 1, -1)