Doubts concerning a general Insert method

Hello, I'm trying to understand a general CRUD class that I've seen here: http://www.phpro.org/classes/PDO-CRUD.html I'm learning PHP and I have some doubts on this method to generally insert data into DB. The class name is crud and here is the method: public function dbInsert($table, $values) { $this->conn(); $fieldnames = array_keys($values[0]); $size = sizeof($fieldnames); $i=1; //construction of the prepared statment $sql = "INSERT INTO $table"; $fields = '( ' . implode(' ,', $fieldnames) . ' )'; $bound = '(:' . implode(', :', $fieldnames) . ' )'; $sql .= $fields.' VALUES '.$bound; //prepares statement e saves it on variable $stmt $stmt = $this->db->prepare($sql); foreach($values as vals) { $stmt->execute($vals); } } To place values on the DB we do: $crud = new crud(); $values = array ( array('animal_name'=>'bruce', 'animal_type'=>'dingo'), array('animal_name'=>'bruce', 'animal_type'=>'kangaroo'), ); $crud->dbInsert('animals', $values); The doubts: 1) Names convention question: Isn't more correct to call $columname, instead of $fieldname ? 2) Why do we have this? $i=1 3) Here: $fieldnames = array_keys($values[0]); We are keeping on variable $fieldnames, the key value of the $values array, when this array is on the position 0 ? And what is *actually* the value returned, considering our array? $values = array ( array('animal_name'=>'bruce', 'animal_type'=>'dingo'), array('animal_name'=>'bruce', 'animal_type'=>'kangaroo'), ); 4) Here: foreach($values as $vals) { $stmt->execute($vals); } We are telling that, for each (line/element/index ???) of $values array, the actual value will be "given"(?) to vals, and the pointer goes to the next (line/element/index)... ? We then execute the prepared statement, but I don't get what are we passing as a param? I mean, what kind of think does the execute PDO method expects as a param? Why $stmt->execute($vals); and not only $stmt->execute(); ? Can I please have your help on clarifying those doubts? Thanks a lot, Márcio