PHP - Help With Session_is_registered

Hi, 

I'm trying to make an old php script (won't work on anything over 5.3) run on php7. I'm not a coder by any means but get along by adapting examples I find on the web. I'm doing OK for the most part but am stuck finding what to replace session_is_registered with.

This is what it is currently: 

$pref = mysql_query("SELECT * FROM preferences WHERE ID = '0'",$connection)
or die(mysql_error());
$pdata = mysql_fetch_array($pref);
mysql_free_result($pref);

// I've already changed the above to:
// 
// pref = mysqli_query($connection,"SELECT * FROM preferences WHERE ID = '0'")
// or die(mysqli_error($connection));
// $pdata = mysqli_fetch_array($pref);
// mysqli_free_result($pref);
//
// Just wanted to show you the whole section in its orignal state.

// It's this line below that I'm not sure about...

if(!session_is_registered('defaultseasonid_') || !session_is_registered('defaultmatchtypeid_') || !session_is_registered('defaultlanguage_'))
	
{
	$_SESSION['defaultseasonid_'] = $pdata['DefaultSeasonID'];
	$_SESSION['defaultmatchtypeid_'] = $pdata['DefaultMatchTypeID'];
	$_SESSION['defaultlanguage_'] = $pdata['DefaultLanguage'];
	$defaultseasonid = $_SESSION['defaultseasonid_'];
	$defaultmatchtypeid = $_SESSION['defaultmatchtypeid_'];
	$defaultlanguage = $_SESSION['defaultlanguage_'];
}
else
{
	$defaultseasonid = $_SESSION['defaultseasonid_'];
	$defaultmatchtypeid = $_SESSION['defaultmatchtypeid_'];
	$defaultlanguage = $_SESSION['defaultlanguage_'];
}

session_unset();

Any pointers anybody can give me would be much appreciated.

TIA