My script:

<?php
  function nullHandler() { return ''; };
  $sha = array();
  $sha['get_all'] = 'nullHandler';

  $xmlDoc = 'xml_file.xml';
  $xslDoc = 'xsl_file.xsl';

  if ($REQUEST_METHOD=='POST') {
    $data = $Keywords;
    $data = ereg_replace(" ", "+", $data);
    $data = ereg_replace("%20", "+", $data);
  }
  elseif ($QUERYSTRING) {
    $data = $Keywords;
    $data = ereg_replace(" ", "+", $data);
    $data = ereg_replace("%20", "+", $data);
  }
  else {
    echo "<b>Missing XML Query &#151; Use form (POST) or querystring (GET)</b>";
    exit;
  }

  echo "Keywords: ".$Keywords."<p>\n";

  $xmlDoc .= "?mkt=de&amp;Keywords=$data";

  echo "XML: ".$xmlDoc."<p>\n";

  $xmlConn = fopen ("$xmlDoc", "r");
  if (!($xmlConn)) {
    echo "<b>Connection to XML feed could not be established!</b>";
    exit;
  }
  else {
    $xmlFeed = fread($xmlConn,20000);
  }
  fclose($xmlConn);

  $xslConn = fopen ("$xslDoc", "r");
  if (!($xslConn)) {
    echo "<b>XSL file could not be found!</b>";
    exit;
  }
  else {
    $xslFeed = fread($xslConn,20000);
  }
  fclose($xslConn);

  $arguments = array('/_xml' => $xmlFeed,'/_xsl' => $xslFeed);

  $xsltproc = xslt_create();

  xslt_set_encoding($xsltproc, "ISO-8859-1");
  xslt_set_scheme_handlers( $xsltproc, $sha );

  $html = xslt_process($xsltproc, 'arg:/_xml', 'arg:/_xsl', NULL, $arguments);
  if (!($html)) die('XSLT processing error: '.xslt_error($xsltproc));
  xslt_free($xsltproc);
  echo $html;
?>

By calling the script above I get the following error:

Fatal error: Call to undefined function: xslt_set_encoding() in /usr/local/apache2/2.0.44/htdocs/overture/transform.php on line 54

Environment:
Linux
Kernel 2.4.19
Apache 2.0.44
PHP 4.3.2RC1 (./configure --with-apxs2=/usr/local/apache2/current/bin/apxs --with-mysql=/usr/local/mysql/current --with-zlib-dir=/usr/lib/ --enable-versioning --enable-track-vars=yes --enable-url-includes --enable-trans-sid --enable-syssvshm=yes --enable-syssvsem=yes --enable-ftp --with-config-file-path=/etc --with-gd --enable-xslt --with-xslt-sablot=/usr/local/sablot --with-expat-dir=/usr/local/expat --enable-javascript --with-sablot-js=/usr)
Expat 1.95.6
Sablotron 0.98
JS 1.5.4-3