What is $transaction == BEGIN_TRANSACTION testing for?

What is $transaction == BEGIN_TRANSACTION testing for?

I'm trying to convert some old code by converting to mysqli. Unfortunately
I can't figure out what part of the code is trying to do, so I don't know
how to change it. It seems to be a standard safety check that everyone who
uses the original mysql extension used, but I can't find anyone who
explains why. Here is the original code:
function query($query = "", $transaction = FALSE)
{
//
// Remove any pre-existing queries
//
unset($this->query_result);
if( $query != "" )
{
$this->num_queries++;
if( $transaction == BEGIN_TRANSACTION && !$this->in_transaction )
{
$result = mysql_query("BEGIN", $this->db_connect_id);
if(!$result)
{
return false;
}
$this->in_transaction = TRUE;
}
$this->query_result = mysql_query($query, $this->db_connect_id);
}
else
{
if( $transaction == END_TRANSACTION && $this->in_transaction )
{
$result = mysql_query("COMMIT", $this->db_connect_id);
}
}
if( $this->query_result )
{
unset($this->row[$this->query_result]);
unset($this->rowset[$this->query_result]);
if( $transaction == END_TRANSACTION && $this->in_transaction )
{
$this->in_transaction = FALSE;
if ( !mysql_query("COMMIT", $this->db_connect_id) )
{
mysql_query("ROLLBACK", $this->db_connect_id);
return false;
}
}
return $this->query_result;
}
else
{
if( $this->in_transaction )
{
mysql_query("ROLLBACK", $this->db_connect_id);
$this->in_transaction = FALSE;
}
return false;
}
}
I can't figure out what they're doing with
if( $transaction == BEGIN_TRANSACTION && !$this->in_transaction )
Can anybody explain this to me?