use php variable within postgresql query

Readers,

Suppose:

$query=$_GET['databasecolumn'[;

$anotherquery=pg_query($databaseconnection, 'SELECT * FROM
databasename WHERE databasecolumnname="'.$query.';"');

Two errors are reported:

PHP Notice:  Undefined index: databasecolumnname

PHP Warning:  pg_query(): Query failed: ERROR:  column ";" does not exist

Any advice please?