Is "PHP" required after "<?"? [duplicate]

Question 1

When PHP parses a file, it looks for opening and closing tags, which are <?php and ?>, which tells PHP to start and stop interpreting the code between them. Parsing in this manner allows PHP to be embedded in all sorts of different documents, as everything outside of a pair of opening and closing tags is ignored by the PHP parser.

PHP also allows for short open tags <? and ?> (which are discouraged because they are only available if enabled with short_open_tag php.ini configuration file directive, or if PHP was configured with the --enable-short-tags option.

Question 2

Its depend on server configuration. If server has enabled short_open_tag

short_open_tag=On

in php.ini file, PHP will allow for short tags.

Note: PHP not recommend to use short tags.

Good Discussion Link

Question 3

If you can use only <?php ?> or you can use both <? ?> and <?php ?> depends on server configuration.

Enabling usage of <? ?> might cause problems in some applications. Imagine, that you have this code in your index.php or index.phtml file:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
    <head></head>
    <body></body>
</html>

With enabled <? ?> you would run into error, because <? is part of xml definition here:

<?xml version="1.0" encoding="UTF-8"?>

Therefore you would have to disable usage of <? ?> in server configuration or you would have to use some workaround.

However, usage of <? ?> makes files where you combine HTML and PHP code more readable. <?= $var ?> is more readable and shorter than <?php echo $var ?>.