How to convert all characters to their html entity equivalent using PHP

Here it goes (assumes UTF-8, but it's trivial to change):

function encode($str) {
    $str = mb_convert_encoding($str , 'UTF-32', 'UTF-8'); //big endian
    $split = str_split($str, 4);

    $res = "";
    foreach ($split as $c) {
        $cur = 0;
        for ($i = 0; $i < 4; $i++) {
            $cur |= ord($c[$i]) << (8*(3 - $i));
        }
        $res .= "&#" . $cur . ";";
    }
    return $res;
}

EDIT Recommended alternative using unpack:

function encode2($str) {
    $str = mb_convert_encoding($str , 'UTF-32', 'UTF-8');
    $t = unpack("N*", $str);
    $t = array_map(function($n) { return "&#$n;"; }, $t);
    return implode("", $t);
}

IMPORTANT

This answer is meant for situations where it's not possible to run/install the 'intl' extension, and only sorts strings by replacing accented characters to non-accented characters. To sort accented characters according to a specific locale, using a Collator is a better approach -- see the other answer to this question for more information.

Sorting by non-accented characters in PHP 5.2

You may try converting both strings to ASCII using iconv() and the //TRANSLIT option to get rid of accented characters;

$str1 = iconv('utf-8', 'ascii//TRANSLIT', $str1);

Then do the comparison

See the documentation here:

http://www.php.net/manual/en/function.iconv.php

[updated, in response to @Esailija's remark] I overlooked the problem of //TRANSLIT translating accented characters in unexpected ways. This problem is mentioned in this question: php iconv translit for removing accents: not working as excepted?

To make the 'iconv()' approach work, I've added a code sample below that strips all non-word characters from the resulting string using preg_replace().

<?php

setLocale(LC_ALL, 'fr_FR');

$names = array(
   'Zoey and another (word) ',
   'Émilie and another word',
   'Amber',
);


$converted = array();

foreach($names as $name) {
    $converted[] = preg_replace('#[^ws]+#', '', iconv('UTF-8', 'ASCII//TRANSLIT', $name));
}

sort($converted);

echo '<pre>'; print_r($converted);

// Array
// (
//     [0] => Amber
//     [1] => Emilie and another word
//     [2] => Zoey and another word 
// )

When opening binary files with fopen(), use the rb mode, ie

$fp = fopen($tmp_name, 'rb');

Alternatively, you may simply use file_get_contents(), eg

$file_content = file_get_contents($tmp_name);

To enable better error reporting, place this at the top of your script

ini_set('display_errors', 'On');
error_reporting(E_ALL);