Drupal theme function override

drupal theme function override
http://drupalsn.com/drupal-tutorials/drupal-theme-function-overriding

"... Drupal can run on different theme engines but as of Drupal 4.7 the phptemplate theme engine became standard, so that’s what we’ll focus on in this tutorial. ...
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i.e. phptemplate is the default theme engine for Drupal
Drupal 預設的 西門(Theme, 版面 樣式 門面 門板 "神門" "西門" ) 引擎為 "phpTemplate"
How about smarty?

theme function === theme_xxx()
"PHP function within Drupal that is prefixed with theme_"


"...By override I mean declaring a new version of a theme function (in template.php) and completely by passing the original theme function in order to alter its output. ...
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