php upgrade - now must set PDO attribute - why?

class DBTable {
        
        private $aDBFields;
        private $aAutoPrimaryField;
        private $aPrimaries;
        private $aData;
        private $table;
 
        function __construct($table,$aData = NULL) {
            global $dbh;
            // first get the list of fields in the database.  You can use this array in your methods to constrain the fields that can be used in insert and update statements.
            $sth = $dbh->prepare("DESCRIBE $table");
            $sth->execute();
			if( $sth->columnCount() ) { 
				while($aRow = $sth->fetch(PDO::FETCH_ASSOC)) {
					$this->aDBFields[] = $aRow['Field'];
					if( $aRow['Extra'] == 'auto_increment' ) {
						$this->aAutoPrimaryField = $aRow['Field'];
					}
					if( $aRow['Key'] == 'PRI' ) {
						$this->aPrimaries[] = $aRow['Field'];
					}
				}
				
				// now store the data passed to the constructor
				if( !is_null($aData) ) {
					foreach($aData as $key => $value) {
						$this->aData[$key] = $value;
					}
				}
				$this->table = $table;
			} else {
                throw new Exception( 'MySQL Error: no such table as '.$table);
			}
        }
 
etc...