html5-php form Submit Get method can submit get value and post method can not get

Why is a PHP form submission get a value that can be submitted in a Get mode and the Post method is not available, it always shows:
notice:undefined index:abc in C:\xampp\htdocs\phpstrom\hello.php on line 2;
This means that the Post method fails to pass the value, not quite understand, the directory is in the C:xampp/htdocs directory.

Post mode:
HTML Source:

    
  
       Document

PHP Source:

Reply content:

Why is a PHP form submission get a value that can be submitted in a Get mode and the Post method is not available, it always shows:
notice:undefined index:abc in C:\xampp\htdocs\phpstrom\hello.php on line 2;
This means that the Post method fails to pass the value, not quite understand, the directory is in the C:xampp/htdocs directory.

Post mode:
HTML Source:

    
  
       Document

PHP Source:

This is just a warning instead of an error generally the workaround is $ABC = isset ($_post[' abc ')? $_post[' abc ']: ';

Look at the request method, put your code on my own test.

You can actually use it in a PHP script first.

var_dump($_POST);

Print the contents of the front-end input to see if it contains the key valueabc

Isset ($_post[' abc ')? $_post[' abc ']: ';

Form form.
Method is post with $_post[' name ']
Method for get is used $_get[' name ']

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