Não, não é, você inseriu o texto Array... se o tipo de coluna $ for texto, seu código deve ler
$tag = array('item1', 'item2', 'item3');
// Prepare a query for execution
$result = pg_prepare($dbconn, "my_query", "INSERT INTO $table ($column) VALUES ($1)");
// Execute the prepared query. Note that it is not necessary to escape
// the string "Joe's Widgets" in any way
foreach( $tag as $i )
$result = pg_execute($dbconn, "my_query", array($i));
/// alternatively you could try this if you really wanna insert a text as array of text without using text[] type - uncomment line below and comment the 2 above
// $result = pg_execute($dbconn, "my_query", array(json_encode($tag)));
ou se você definiu $column como text[] que é legal no postgresql como array, o código deve ler
$tag = array('item1', 'item2', 'item3');
// Prepare a query for execution
$result = pg_prepare($dbconn, "my_query", "INSERT INTO $table ($column) VALUES ($1)");
// Execute the prepared query. Note that it is not necessary to escape
// the string "Joe's Widgets" in any way
$tmp = json_encode($tag);
$tmp[0] = '{';
$tmp[strlen($tmp) - 1] = '}';
$result = pg_execute($dbconn, "my_query", array($tmp));
