Look at the function in wp-includes/post-template.php. There you see what information you can get:
apply_filters(
'wp_get_attachment_link'
, "<a href="https://wordpress.stackexchange.com/questions/16809/$url" title="$post_title">$link_text</a>"
, $id
, $size
, $permalink
, $icon
, $text
);
Note that you cannot access the $link_text and the $_post object as a standalone variables. Bug? Bug!
In your filter you cannot change the order of the arguments, just the number.
So add_filter( 'wp_get_attachment_link', 'modify_attachment_link', 10, 2 ); will give you the link markup and $id. The highest number of available arguments is 6.
The return value of your function will replace the first argument.
An (untested) example for changing the link URL:
/**
* Replaces the URL for an attachment link.
*
* @param string $markup Original link markup
* @param int $id Post id
* @param mixed $size Image size, array or string
* @param string $permalink URL
* @param bool $icon Use an icon?
* @param bool $text Use text?
* @return string New markup
*/
function modify_attachment_link( $markup, $id, $size, $permalink, $icon, $text )
{
// We need just thumbnails.
if ( 'thumbnail' !== $size )
{ // Return the original string untouched.
return $markup;
}
// We have stored the new URL in a post meta field.
// See https://wordpress.stackexchange.com/q/3097 for an example.
$new_url = get_post_meta( $id, 'extra_url', TRUE );
if ( empty ( $new_url ) )
{ // There is no URL.
return $markup;
}
// Recreate the missing information.
$_post = & get_post( $id );
$post_title = esc_attr( $_post->post_title );
if ( $text )
{
$link_text = esc_attr( $text );
}
elseif (
( is_int( $size ) && $size != 0 )
or ( is_string( $size ) && $size != 'none' )
or $size != FALSE
)
{
$link_text = wp_get_attachment_image( $id, $size, $icon );
}
else
{
$link_text="";
}
if ( trim( $link_text ) == '' )
{
$link_text = $_post->post_title;
}
return "<a href="https://wordpress.stackexchange.com/questions/16809/$new_url" title="$post_title">$link_text</a>";
}
add_filter( 'wp_get_attachment_link', 'modify_attachment_link', 10, 6 );
Further reading: