Contents within a pair of opening and closing tag are interpreted by php parser.

Php has three types of opening and closing tags.
1.Standard tags

 2.Short echo tag

 here, 

Based on PHP tag availability you may use the standard   tags or short-echo  tags but this short tags in particular should/may not be used because it is usually disabled by most system administrators  via the short_open_tag php.ini configuration file directive, or are disabled by default if PHP is configured with the --disable-short-tags setup.
 
Assuming a file contents are only PHP codes, it is more preferable to omit the PHP closing tag at the file end.
Doing this prevents unusual white space or new lines added after the PHP closing tag

These are called PHP's opening  and any code or contents in-between these two is likely to be interpreted by the php parser.

All contents be it code or block of state inside these tags can be placed in HTML document, so that inserted code is executed on server, while the remainings of the document processed on client browser's HTML parser.

Note that the file document that contains the html must be saved as .php file

PHP parses a file to basically search for the PHP open and close tags, which are . It is only this tags that instructs the PHP where to start and stop interpreting php codes within them.

PHP files may sometimes consist of HTML tags, and some PHP scripting code. 

A PHP script can be inserted any place in the PHP document.

The file default extension for PHP documents is “.php”.

below I Presented an example of a common PHP file with html tags and a PHP tags(scripts) as well, I demonstrated the of uses PHP in-built function called  “echo”  to display this content “Hi programmer!” on browser when ran.

Php tag example

Yes you can, study the example I gave you above.

So far you use any HTML you want outside and separate from the PHP tags. 

Vice versa, assuming you've place the PHP code in HTML code script, just write the PHP wherever you want all the time just make sure that the php code is inside the PHP tags as instructed below for php documents.

A PHP document begins with

if need may.

This php tags tell the web server to handle everything within the tags as PHP code when run. 

PHP The code is very simple. 

It uses some in-build PHP functions for example “echo” the one we used to display the text “Hi Programmer” in example above. 

Hence you know that everything outside these tags is interpreted by html parser on the browser when ran.

A PHP code scripts are text files with .php extension stored on web/host server. 

The PHP parser on server searches for logics and characters between . 

are called The PHP's opening and closing tags.

These set of tags are used to mark the beginning and end of a block of code, in between which any amount of code can be written.

PHP tag can be embedded in an HTML document and is placed anywhere on the HTML document page but the file extension must .php. 

For your information if you are told that this tag  works in php well it does not work for me.

I only use this