如何在PHP中使用变量作为字符串？

I am pretty new to PHP and am trying to use variables in a string, here is my code:

<?php
    $userName = $_GET['Username'];
    $message = $_GET['Message'];

    if($_POST['Submit']){ 
        $open = fopen("new.txt",'a'); 
        $text = "
________________________________________"+$userName+":"+$message+"
________________________________________";
        fwrite($open, $text); 
        fclose($open); 
        echo "File updated.<br />";  
        echo "File:<br />"; 
        $file = file("new.txt"); 
        foreach($file as $text) { 
            echo $text."<br />"; 
        }
    }
?>

Also, would I use 'a' for adding to a file? If not what would I use? Please help me, thanks in advance!

Use . instead of + for concatenation.

Or, you can just do:

$text = "
________________________________________$userName:$message
________________________________________";

or (to be slightly clearer to the parser):

$text = "
________________________________________{$userName}:{$message}
________________________________________";

$text = " ___$userName:$message " will work. Also $text = "whatever{$var}bar{$var2}". The documentation for fopen is available here and says of 'a':

Open for writing only; place the file pointer at the end of the file.

'a' in this context generally means 'append'.

$text = "
________________________________________".$userName.":".$message."
________________________________________";

Use . for concatination

a would append to a file, so u want to replace the content you would use w

I always like single quotes (when possible) cause they are faster:

$str = '5+5='.(5+5);

Also comma for echo instead of concatenation cause is faster:

echo '5+5=',(5+5);

And in very complex strings (if don't care for speed) and also quotes needed I use variables to make things look more clear:

 $dbq="\"";
 $sq='\'';
 echo $dbq,'This is in double quotes.',$dbq,'<br>';
 echo $sq,'This is in single quotes.',$sq;