How to determine whether a user is logged on?

Determine whether a user is logged on & lt ;? Phpsession_start (); if ($ _ SESSION ['username']) {echo & quot; & lt; ahref = 'http: // www.kolo.net.cn/shop/index.php' &g determine whether a user is logged on


This code, session_start (); does not play a role. I wrote it into the header file. what I'm depressed about is that the session can be used on the background code page, why can't I use it in the front-end page code ..

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Is there any output above? Does the file format have a BOM header?