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.compassword
// $eLEADID - lead id to convert


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

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

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

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

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

Sample Output

stdClass Object
(
    [result] => stdClass Object
        (
            [accountId] => 0015000000YiUUBAA3
            [contactId] => 0035000000o1ZJmAAM
            [leadId] => 00Q5000000VbEY7EAN
            [opportunityId] => 
            [success] => 1
        )

)