Get post content intro text on category.php?

You’re using an old version of WordPress. Upgrade to 3.5 and you will no longer see that error.

Here’s the function declaration for get_post in WP 3.4:

function &get_post(&$post, $output = OBJECT, $filter="raw")

Notice the ampersand in front of $post? That would mean the value is passed by reference. You can give the function a literal, it has to be the variable.

The correct solution is to update to WP 3.5 (we’re on 3.5.1 at the time of writing). Get post to WP 3.5+:

function get_post( $post = null, $output = OBJECT, $filter="raw" )

The less good way is to simply cheat when you call the function and do an assignment + call at the same time:

$a_post = get_post($a_post_id = 439);

$a_post_id = 439 in the function arguments creates the $a_post_id variable and pass it into the function.