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Collection::count — Get document count
   This functionality is similar to a SELECT COUNT(*) SQL 
   operation against the MySQL server for the current schema and collection.
   In other words, it counts the number of documents in the collection.
  
This function has no parameters.
The number of documents in the collection.
Example #1 mysql_xdevapi\Collection::count() example
<?php
$session = mysql_xdevapi\getSession("mysqlx://user:password@localhost");
$session->sql("DROP DATABASE IF EXISTS addressbook")->execute();
$session->sql("CREATE DATABASE addressbook")->execute();
$schema = $session->getSchema("addressbook");
$create = $schema->createCollection("people");
$collection = $schema->getCollection("people");
$result = $collection
  ->add(
  '{"name": "Bernie",
    "jobs": [
      {"title":"Cat Herder","Salary":42000}, 
      {"title":"Father","Salary":0}
    ],
    "hobbies": ["Sports","Making cupcakes"]}',
  '{"name": "Jane",
    "jobs": [
      {"title":"Scientist","Salary":18000}, 
      {"title":"Mother","Salary":0}
    ],
    "hobbies": ["Walking","Making pies"]}')
  ->execute();
var_dump($collection->count());
?>
The above example will output:
int(2)