PHP convert string to hex and hex to string

I got the problem when convert between this 2 type in PHP. This is the code I searched in google

function strToHex($string){
    $hex='';
    for ($i=0; $i < strlen($string); $i++){
        $hex .= dechex(ord($string[$i]));
    }
    return $hex;
}
 
 
function hexToStr($hex){
    $string='';
    for ($i=0; $i < strlen($hex)-1; $i+=2){
        $string .= chr(hexdec($hex[$i].$hex[$i+1]));
    }
    return $string;
}

I check it and found out this when I use XOR to encrypt.

I have the string "this is the test", after XOR with a key, I have the result in string ↕↑↔§P↔§P ♫§T↕§↕. After that, I tried to convert it to hex by function strToHex() and I got these 12181d15501d15500e15541215712. Then, I tested with the function hexToStr() and I have ↕↑↔§P↔§P♫§T↕§q. So, what should I do to solve this problem? Why does it wrong when I convert this 2 style value?

For people that end up here and are just looking for the hex representation of a (binary) string.

bin2hex("that's all you need");
# 74686174277320616c6c20796f75206e656564

hex2bin('74686174277320616c6c20796f75206e656564');
# that's all you need

Doc: bin2hex, hex2bin.

For any char with ord($char) < 16 you get a HEX back which is only 1 long. You forgot to add 0 padding.

This should solve it:

<?php
function strToHex($string){
    $hex = '';
    for ($i=0; $i<strlen($string); $i++){
		$ord = ord($string[$i]);
		$hexCode = dechex($ord);
        $hex .= substr('0'.$hexCode, -2);
    }
    return strToUpper($hex);
}
function hexToStr($hex){
    $string='';
    for ($i=0; $i < strlen($hex)-1; $i+=2){
        $string .= chr(hexdec($hex[$i].$hex[$i+1]));
    }
    return $string;
}


// Tests
header('Content-Type: text/plain');
function test($expected, $actual, $success) {
	if($expected !== $actual) {
		echo "Expected: '$expected'\n";
		echo "Actual:   '$actual'\n";
		echo "\n";
		$success = false;
	}
	return $success;
}

$success = true;
$success = test('00', strToHex(hexToStr('00')), $success);
$success = test('FF', strToHex(hexToStr('FF')), $success);
$success = test('000102FF', strToHex(hexToStr('000102FF')), $success);
$success = test('↕↑↔§P↔§P ♫§T↕§↕', hexToStr(strToHex('↕↑↔§P↔§P ♫§T↕§↕')), $success);

echo $success ? "Success" : "\nFailed";

PHP :

string to hex:

implode(unpack("H*", $string));

hex to string:

pack("H*", $hex);

Here's what I use:

function strhex($string) {
  $hexstr = unpack('H*', $string);
  return array_shift($hexstr);
}

function hexToStr($hex){
    // Remove spaces if the hex string has spaces
	$hex = str_replace(' ', '', $hex);
	return hex2bin($hex);
}
// Test it 
$hex    = "53 44 43 30 30 32 30 30 30 31 37 33";
echo hexToStr($hex); // SDC002000173

/**
 * Test Hex To string with PHP UNIT
 * @param  string $value
 * @return 
 */
public function testHexToString()
{
	$string = 'SDC002000173';
	$hex    = "53 44 43 30 30 32 30 30 30 31 37 33";
	$result = hexToStr($hex);

    $this->assertEquals($result,$string);
}

Using @bill-shirley answer with a little addition

function str_to_hex($string) {
$hexstr = unpack('H*', $string);
return array_shift($hexstr);
}
function hex_to_str($string) {
return hex2bin("$string");
}

Usage:

  $str = "Go placidly amidst the noise";
  $hexstr = str_to_hex($str);// 476f20706c616369646c7920616d6964737420746865206e6f697365
  $strstr = hex_to_str($str);// Go placidly amidst the noise

I only have half the answer, but I hope that it is useful as it adds unicode (utf-8) support

	/**
     * hexadecimal to unicode character
	 * @param  string  $hex
	 * @return string
	 */
    function hex2uni($hex) { 
      $dec = hexdec($hex);
		if($dec < 128) {
			return chr($dec);
		}
		if($dec < 2048) {
			$utf = chr(192 + (($dec - ($dec % 64)) / 64));
		} else {
			$utf = chr(224 + (($dec - ($dec % 4096)) / 4096));
			$utf .= chr(128 + ((($dec % 4096) - ($dec % 64)) / 64));
		}
		return $utf . chr(128 + ($dec % 64));
    }

To string

    var_dump(hex2uni('e641'));

Based on: http://www.php.net/manual/en/function.chr.php#Hcom55978

You can try the following code to convert the image to hex string

<?php
$image = 'sample.bmp';
$file = fopen($image, 'r') or die("Could not open $image");
while ($file && !feof($file)){
$chunk = fread($file, 1000000); # You can affect performance altering
this number. YMMV.
# This loop will be dog-slow, almost for sure...
# You could snag two or three bytes and shift/add them,
# but at 4 bytes, you violate the 7fffffff limit of dechex...
# You could maybe write a better dechex that would accept multiple bytes
# and use substr... Maybe.
for ($byte = 0; $byte < strlen($chunk); $byte++)){
echo dechex(ord($chunk[$byte]));
}
}
?>