shorter way of echoing a variable in php?

In the last two days, ive run across code that has php echo'd variables displayed like this

<?=$selected?>开发者_开发问答

What exactly is going on and why? What is this called?

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That's called a Short Tag. It's a shortcut to <?php echo $selected;?>. It is widely adopted, but there's a lot of literature out there that does not recommend its use as it leads to less portable code (many PHP installations do not have short tags enabled). I happen to agree, just take a look at this user's woes. Also, see:

Are PHP Short Tags acceptable to use?