Editing a text file from plugin menu

Question

This is my first time designing a plugin, so I would appreciate any assistance that you all can provide.

My plugin has a txt file with datetime objects in it. The goal is to read the txt file from a plugin admin page, allow users to edit it, and then save the file once again.

I have no problem reading the file, but I am unable to save it. When I click the button to save it, the browser redirects me somewhere else and doesn't end up saving my file.

EDIT: I should add - this code worked just fine outside of wordpress (as its own PHP page). I can't seem to get it to work with wordpress.

Here is my code. I would greatly appreciate any advice:

if($_POST['addition']){ 
    $file_open = fopen("dates.txt","w+");
    fwrite($file_open, $_POST['addition']);
    fclose($file_open);
    }

echo '<form action="<?=$PHP_SELF?>" method="POST">
    <textarea name="addition" COLS=50 ROWS=20>';
    $datalines = file ("dates.txt");
        foreach ($datalines as $zz) {
            echo $zz;
        }                   
echo '</textarea><input type="submit" name="button" value="Save Evenings Before Solemnities File"></form>';
            echo '</td>';

Answer 1

First of all put these all code inside in init hook, and use plugin_dir_path() func to get plugin dir path.

add_action('init',function(){

    $plugindir = plugin_dir_path( __FILE__ );

    if(isset($_POST['addition'])){ 

        $file_open = fopen($plugindir."dates.txt","w+");
        fwrite($file_open, $_POST['addition']);
        fclose($file_open);
    }
        echo '<form action="'.$_SERVER['PHP_SELF'].'" method="POST">
        <textarea name="addition" COLS=50 ROWS=20>';

        $datalines = file ($plugindir."dates.txt");

        foreach ($datalines as $zz) {
            echo $zz;
        }                   
        echo '</textarea><input type="submit" name="button" value="Save Evenings Before Solemnities File"></form>';
        echo '</td>';
        exit;

});

Try this above code.