How to take the post value in the same PHP file

How to take the post value in the same PHP file
The code assumes that

 
 if ($_post) {
$a = $_post[' a '];
echo "$a";   can be output normally.
}else{
$a = $_post[' a '];
echo "$a";   Cannot output value!!!!!
}
?>

Excuse me, how can I get the value of $_post[' a ' in else? to seek solutions.
------to solve the idea----------------------
References:

Quote: References:

Talk about your specific needs in order to better answer your questions. What you said is completely unclear.

is to fill out a content on the post page, and then submit it on a non-post page to get this content

stored in the session, in the database, in the file, etc.

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