Sample

<?php
// SOAP_CLIENT_BASEDIR - folder that contains the PHP Toolkit and your WSDL
// $USERNAME - variable that contains your Salesforce.com username (must be in the form of an email)
// $PASSWORD - variable that contains your Salesforce.com password

define("SOAP_CLIENT_BASEDIR", "../../soapclient");
require_once (SOAP_CLIENT_BASEDIR.'/SforcePartnerClient.php');
require_once (SOAP_CLIENT_BASEDIR.'/SforceHeaderOptions.php');

require_once ('../userAuth.php');

try {
  $mySforceConnection = new SforcePartnerClient();
  $mySoapClient = $mySforceConnection->createConnection(SOAP_CLIENT_BASEDIR.'/partner.wsdl.xml');
  $mylogin = $mySforceConnection->login($USERNAME, $PASSWORD);

  $leadConvert = new stdClass;
  $leadConvert->convertedStatus='Closed - Converted';
  $leadConvert->doNotCreateOpportunity=false;
  $leadConvert->leadId=$convertLEADID;
  $leadConvert->overwriteLeadSource=true;
  $leadConvert->sendNotificationEmail=true;

  $leadConvertArray = array($leadConvert);
  $leadConvertResponse = $mySforceConnection->convertLead($leadConvertArray);

  print_r($leadConvertResponse);
} catch (Exception $e) {
  echo $mySforceConnection->getLastRequest();
  echo $e->faultstring;
}
?>

Sample Output

stdClass Object
(
    [result] => stdClass Object
        (
            [accountId] => 0015000000YiUUVAA3
            [contactId] => 0035000000o1ZKGAA2
            [leadId] => 00Q5000000VbEZOEA3
            [opportunityId] => 0065000000FysHyAAJ
            [success] => 1
        )

)