Fournir une instruction préparée avec un tableau

Non, ce n'est pas le cas, vous avez inséré le texte Array... si le type de $column est du texte, votre code doit lire

$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 si vous avez défini $column comme texte[] qui est légal dans postgresql comme tableau, le code doit lire

$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));