Error in WordPress affecting site editing
CiviCRM 4.2.4 is a very old version - it's from 2012.  It's not compatible with newer versions of PHP - I'd be surprised if it worked with PHP 5.6, let alone 7+.  You can downgrade PHP or upgrade CiviCRM - but since you're upgrading from such an old version, be sure to a) take a backup of the database before beginning, b) upgrade a few versions at a time (I'd go to the last version of 4.4, 4.5, 4.6, 4.7 before going to 5.18), c) if you DO have to restore the database, be sure to delete the existing database before restoring the backup.