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The $wgAppleTouchIcon config var can specify the path to an icon to be used for home-screen bookmarks on iOS devices.

Originally there was only a single resolution required, 57x57 pixels for the classic iPhone and iPod touch, so only a single link was needed.

Newer devices with larger or sharper screens prefer other resolutions: 72x72 for iPad, and 114x114 for the iPhone 4 and newer iPod Touch models with high-resolution displays. Under iOS 4.2, CSS3 multiple sizes may be specified by providing multiple links, with the 'sizes' attribute listing width & height.

Recommend allowing $wgAppleTouchIcon to be an array, perhaps:

$wgAppleTouchIcon = array(
    'default' => '/iphone-57.png',
    '72x72' => '/iphone-72.png',
    '114x114' => '/iphone-114.png',
);

or a more device-name-friendly:

$wgAppleTouchIcon = array(
    'iPhone' => '/iphone-57.png',
    'iPad' => '/iphone-72.png',
    'iPhone 4' => '/iphone-114.png',
);