SQLSTATE[HY000] [2002] No such file or directory

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<b>Fatal error</b>:  Uncaught Error: Call to a member function prepare() on null in /usr/share/php/PDODatabase.php:51
Stack trace:
#0 /var/www/forms/nurs/trad/inc/header.php(6): PDODatabase-&gt;query()
#1 /var/www/forms/nurs/trad/index.php(6): require('/var/www/forms/...')
#2 {main}
  thrown in <b>/usr/share/php/PDODatabase.php</b> on line <b>51</b><br />