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Hooks work similar to the callback functions of the Data Container Array. You can register one or more functions for a certain event and when the event is triggered, the callback functions are executed. Hooks allow you to add custom functionality to the core.
// config.php
$GLOBALS['TL_HOOKS']['getImage'][] = array('MyClass', 'myGetImage');
// MyClass.php
public function myGetImage($image, $width, $height, $mode, $strCacheName, $objFile, $target)
{
return MyImage::generateThumbnail($image, $widht, $height, $mode);
}