You're right - the docs are a bit sparse on the PHP Metadata Client. Here is some sample code to create a custom object and a custom field on that object. Note that you will need to get the very latest PHP toolkit from https://github.com/developerforce/Force.com-Toolkit-for-PHP since I fixed a bug in creating custom fields just a few minutes ago.

<?php
// Change this to point to wherever you've put the soapclient directory
$soapclient = '../Force.com-Toolkit-for-PHP/soapclient';

require_once ($soapclient.'/SforcePartnerClient.php');
require_once ($soapclient.'/SforceMetadataClient.php');

// I use environment variables for credentials - this is good practice
// You can just put your username/password instead if you like
define("SF_SECURITY_TOKEN", getenv("SECURITY_TOKEN"));
define("SF_USERNAME", getenv("USERNAME"));
define("SF_PASSWORD", getenv("PASSWORD"));

define("SF_PARTNER_WSDL", $soapclient.'/partner.wsdl.xml');
define("SF_METADATA_WSDL", $soapclient.'/metadata.wsdl.xml');

// "" or "namespace__"
define("SF_NAMESPACE_PREFIX", getenv("NAMESPACE_PREFIX"));

$mySforceConnection = new SforcePartnerClient();
$mySoapClient = $mySforceConnection->createConnection(SF_PARTNER_WSDL);
$myLoginResult = $mySforceConnection->login(SF_USERNAME, 
  SF_PASSWORD.SF_SECURITY_TOKEN);

$myMetadataConnection = new SforceMetadataClient(SF_METADATA_WSDL, 
  $myLoginResult, $mySforceConnection);

$customObject = new SforceCustomObject();
$customObject->setFullName(SF_NAMESPACE_PREFIX.'ObjFromPHP__c');
$customObject->setDeploymentStatus(DEPLOYMENT_STATUS_DEPLOYED);
$customObject->setDescription("Custom Created Object from the Metadata API");
$customObject->setEnableActivities(true);
$customObject->setEnableDivisions(false);
$customObject->setEnableHistory(true);
$customObject->setEnableReports(true);
$customObject->setHousehold(false);
$customObject->setLabel("Custom Obj from PHP");
$customObject->setPluralLabel('My Custom Objs from PHP');
$customObject->setSharingModel(SHARING_MODEL_READWRITE);

$customField = new SforceCustomField();
$customField->setDescription('Description of name field');
$customField->setLabel('My Name Field Label');
$customField->setType('Text');

$customObject->setNameField($customField);

$customObjectResult = $myMetadataConnection->create($customObject);

echo "Created custom object - result:\n";
print_r($customObjectResult);

$sleep = 1;
while (!$customObjectResult->result->done) {
  sleep($sleep);
  $sleep *= 2;
  
  $customObjectResult = $myMetadataConnection->checkStatus(array($customObjectResult->result->id));  
  echo "Checked status - result:\n";
  print_r($customObjectResult);
}

if ($customObjectResult->result->state == 'Error') {
  // You might want to do something other than die()
  die("Error creating custom object: ".$customObjectResult->result->statusCode.
    " ".$customObjectResult->result->message."\n");
}

echo "Custom object created successfully\n";

$customField1 = new SforceCustomField();
$customField1->setFullName(SF_NAMESPACE_PREFIX.'ObjFromPHP__c.CustomFieldb__c');
$customField1->setDescription('Description of custom field');
$customField1->setLabel('My Custom Field Label');
$customField1->setType('Text');
$customField1->setLength('10');

$customFieldResult = $myMetadataConnection->create($customField1, 'CustomField');

echo "Created custom field - result:\n";
print_r($customFieldResult);

$sleep = 1;
while (!$customFieldResult->result->done) {
  sleep($sleep);
  $sleep *= 2;
  
  $customFieldResult = $myMetadataConnection->checkStatus(array($customFieldResult->result->id));  
  echo "Checked status - result:\n";
  print_r($customFieldResult);
}

if ($customFieldResult->result->state == 'Error') {
  // You might want to do something other than die()
  die("Error creating custom field: ".$customFieldResult->result->statusCode.
    " ".$customFieldResult->result->message."\n");
}

echo "Custom field created successfully\n";
?>