对于在这里结束并且只是寻找（二进制）字符串的十六进制表示的人。

bin2hex("that's all you need");
# 74686174277320616c6c20796f75206e656564
hex2bin('74686174277320616c6c20796f75206e656564');
# that's all you need

文件编号：bin2hex，hex2bin。

对于ord（$char）<16的任何字符，您将得到一个长度仅为1的十六进制back。您忘记添加0填充。
这应该可以解决：

<?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：
字符串到十六进制：

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

十六进制到字符串：

pack("H*", $hex);

我用的是：

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);
}

使用@bill shirley answer加上一点附加

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

用法：

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

我只有一半的答案，但我希望它是有用的，因为它添加了unicode（utf-8）支持

//decimal to unicode character
function unichr($dec) { 
  if ($dec < 128) { 
    $utf = chr($dec); 
  } else if ($dec < 2048) { 
    $utf = chr(192 + (($dec - ($dec % 64)) / 64)); 
    $utf .= chr(128 + ($dec % 64)); 
  } else { 
    $utf = chr(224 + (($dec - ($dec % 4096)) / 4096)); 
    $utf .= chr(128 + ((($dec % 4096) - ($dec % 64)) / 64)); 
    $utf .= chr(128 + ($dec % 64)); 
  } 
  return $utf;
}

到字符串

var_dump(unichr(hexdec('e641')));

资料来源：http://www.php.net/manual/en/function.chr.php#hcom55978

您可以尝试以下代码将图像转换为十六进制字符串

<?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]));
}
}
?>