如何使用PHP在特定路径上保存* .txt文件？

Question

I am trying to write a simple PHP script that writes a string to .txt file. Simple. For that I have this:

<?php
if(isset(isset($_GET["fileName"]) == true && isset($_GET["output"]) == true){
    $fp = fopen($_GET["fileName"], "a+");
    if($fp !== null){
        fputs($fp, $_GET["output"] . "\r\n");
        fclose($fp);
    }
}
?>

But I really need to be able to save to a specific location, so I have written this:

<?php
if(isset($_GET["filePath"]) == true && isset($_GET["fileName"]) == true && isset($_GET["output"]) == true){
    $fp = fopen($_GET["filePath"] . $_GET["fileName"], "a+");
    if($fp !== null){
        fputs($fp, $_GET["output"] . "\r\n");
        fclose($fp);
    }
}
?>

But of course, this doesn't work.
1. How can I save to a specific location with PHP?
2. Can this path be a relative path? (i just send "/output/" and the save would be done as expected).

Thank you.

Now I am here:

<?php
if(isset($_GET["filePath"]) && isset($_GET["fileName"]) && isset($_GET["output"])){

    $filename=preg_replace('/[^a-z0-9]/i', '', $_GET['fileName']).'.txt';
    $filepathSanitized='http://thepathtomywebsite/~subdomain/'.$_GET["filePath"].$filename;

    echo $filepathSanitized;  // i get the correct path, but no file is present there 
    $fp = fopen($_GET["filepathSanitized"], "a+");
    if($fp !== null){

        fputs($fp, $_GET["output"] . "\r\n");
        fclose($fp);
    }
    else echo ('Something wrong'); // it never gets to this
}
?>

And i am calling the above like this:

$.get("save_me.php", { filePath: "demo/", fileName: "text", output: 'thing to write into' });

But no file is created. I don't know what to debug to find out what goes wrong. Thank you.

Answer 1

You will probably find this is a permissions issue - debug your code to see if $fp is actually set.

Also, code like this is potentially very dangerous, allowing a file to be created or replaced anywhere the webserver is allowed to access. You should sanitize the fileName, and create the full path yourself, eg

//clean any nasties from the filename
$filename=preg_replace('/[^a-z0-9]/i', '', $_GET['fileName']).'.txt';

//calculate a path relative to our document root...
$filepath=$_SERVER['DOCUMENT_ROOT'].'/path/to/files/'.$filename;

Answer 2

But no file is created. I don't know what to debug to find out what goes wrong. Thank you.

You should debug the argument that is passed to fopen().