The fact that MS-word and some other sources use CP-1252, and that it is so close to Latin1 ('ISO-8859-1') causes a lot of confusion. What confused me the most was finding that mySQL uses CP-1252 by default.

You may run into trouble if you find yourself tempted to do something like this:
<?php
    $trans[chr(149)] = '&bull;';    // Bullet
    $trans[chr(150)] = '&ndash;';    // En Dash
    $trans[chr(151)] = '&mdash;';    // Em Dash
    $trans[chr(152)] = '&tilde;';    // Small Tilde
    $trans[chr(153)] = '&trade;';    // Trade Mark Sign
?>

Don't do it. DON'T DO IT!

You can use:
<?php
    $translationTable = get_html_translation_table(HTML_ENTITIES, ENT_NOQUOTES, 'WINDOWS-1252');
?>

or just convert directly:
<?php
    $output = htmlentities($input, ENT_NOQUOTES, 'WINDOWS-1252');
?>

But your web page is probably encoded UTF-8, and you probably don't really want CP-1252 text flying around, so fix the character encoding first:
<?php
    $output = mb_convert_encoding($input, 'UTF-8', 'WINDOWS-1252');
    $ouput = htmlentities($output);
?>

Be careful using get_html_translation_table() in a loop, as it's very slow.

to display the mapping on a webpage no matter what the server encoding is, this can be used

  echo "<pre>\n";
  echo htmlentities(print_r((get_html_translation_table(HTML_SPECIALCHARS)), true));
  echo htmlentities(print_r((get_html_translation_table(HTML_ENTITIES)), true));

since get_html_translation_table() actually gives the special chars in iso-8859-1 (Latin-1) encoding, so to see the tables correctly using

  print_r(get_html_translation_table(HTML_ENTITIES));

your server needs to give a HTTP header as iso-8859-1, unless you use header() or manually set the browser's encoding setting to iso-8859-1.  And you need to view the source of the page to see the mapping.  (except English version of IE 7 outputs the page source as iso-8859-1 anyway).

get_html_translation_table
It works only with the first 256 Codepositions.
For Higher Positions, for Example ф
(a kyrillic Letter) it shows the same.

htmlentities includes htmlspecialchars, so here's how to convert an UTF-8 string :
htmlentities($string, ENT_QUOTES, 'UTF-8');

I wrote a quick little function for converting something like '·' into '·':

$to_convert = '·'; 
$table = get_html_translation_table(HTML_ENTITIES);
$equiv = '&#'.ord(array_search($to_convert,$table)).';';

Not sure what's going on here but I've run into a problem that others might face as well...

<?php

$translations = array_flip(get_html_translation_table(HTML_ENTITIES,ENT_QUOTES));

?>

returns the single quote ' as being equal to &#39; while

<?php

$translatedString = htmlentities($string,ENT_QUOTES);

?>
returns it as being equal to &#039;

I've had to do a specific string replacement for the time being... Not sure if it's an issue with the function or the array manipulation.

-Pat

If you have troubles (like me) getting data from ISO-8859-1 encoded forms where user copy and paste from word, this routine could be useful.
It adds to the standard get_html_translation_table the codes of the characters usually M$ Word replacs into typed text.
Otherwise those characters would never be displayed correctly in html output.

function get_html_translation_table_CP1252() {
    $trans = get_html_translation_table(HTML_ENTITIES);
    $trans[chr(130)] = '‚';    // Single Low-9 Quotation Mark
    $trans[chr(131)] = 'ƒ';    // Latin Small Letter F With Hook
    $trans[chr(132)] = '„';    // Double Low-9 Quotation Mark
    $trans[chr(133)] = '…';    // Horizontal Ellipsis
    $trans[chr(134)] = '†';    // Dagger
    $trans[chr(135)] = '‡';    // Double Dagger
    $trans[chr(136)] = 'ˆ';    // Modifier Letter Circumflex Accent
    $trans[chr(137)] = '‰';    // Per Mille Sign
    $trans[chr(138)] = 'Š';    // Latin Capital Letter S With Caron
    $trans[chr(139)] = '‹';    // Single Left-Pointing Angle Quotation Mark
    $trans[chr(140)] = 'Π   ';    // Latin Capital Ligature OE
    $trans[chr(145)] = '‘';    // Left Single Quotation Mark
    $trans[chr(146)] = '’';    // Right Single Quotation Mark
    $trans[chr(147)] = '“';    // Left Double Quotation Mark
    $trans[chr(148)] = '”';    // Right Double Quotation Mark
    $trans[chr(149)] = '•';    // Bullet
    $trans[chr(150)] = '–';    // En Dash
    $trans[chr(151)] = '—';    // Em Dash
    $trans[chr(152)] = '˜';    // Small Tilde
    $trans[chr(153)] = '™';    // Trade Mark Sign
    $trans[chr(154)] = 'š';    // Latin Small Letter S With Caron
    $trans[chr(155)] = '›';    // Single Right-Pointing Angle Quotation Mark
    $trans[chr(156)] = 'œ';    // Latin Small Ligature OE
    $trans[chr(159)] = 'Ÿ';    // Latin Capital Letter Y With Diaeresis
    ksort($trans);
    return $trans;
}

without heavy scientific analysis, this seems to work as a quick fix to making text originating from a Microsoft Word document display as HTML:

<?php
function DoHTMLEntities ($string)
    {
        $trans_tbl = get_html_translation_table (HTML_ENTITIES);
        
        // MS Word strangeness.. 
        // smart single/ double quotes:
        $trans_tbl[chr(145)] = '\''; 
        $trans_tbl[chr(146)] = '\''; 
        $trans_tbl[chr(147)] = '&quot;'; 
        $trans_tbl[chr(148)] = '&quot;'; 

                // Acute 'e'
        $trans_tbl[chr(142)] = '&eacute;';
        
        return strtr ($string, $trans_tbl);
    }
?>

If you want to display special HTML entities in a web browser, you can use the following code:

<?
$entities = get_html_translation_table(HTML_ENTITIES);
foreach ($entities as $entity) {
    $new_entities[$entity] = htmlspecialchars($entity);
}
echo "<pre>";
print_r($new_entities);
echo "</pre>";
?>

If you don't, the key name of each element will appear to be the same as the element content itself, making it look mighty stupid. ;)

Alans version didn't seem to work right. If you're having the same problem consider using this slightly modified version instead:

function unhtmlentities ($string)  {
   $trans_tbl = get_html_translation_table (HTML_ENTITIES);
   $trans_tbl = array_flip ($trans_tbl);
   $ret = strtr ($string, $trans_tbl);
   return preg_replace('/&#(\d+);/me', 
      "chr('\\1')",$ret);
}