Be warned, that is_writable returns false for non-existent files, although they can be written to the queried path.

In Linux, you might encountering an issue which is a file is not writable even tho it has 644 permission! The problem is with SELinux, just disable it or add rules to allow it.

To Darek and F Dot: About group permissions, there is this note in the php.ini file:
; By default, Safe Mode does a UID compare check when
; opening files. If you want to relax this to a GID compare,
; then turn on safe_mode_gid.
safe_mode_gid = Off

Check director is writable recursively. to return true, all of directory contents  must be writable

<?php
function is_writable_r($dir) {
    if (is_dir($dir)) {
        if(is_writable($dir)){
            $objects = scandir($dir);
            foreach ($objects as $object) {
                if ($object != "." && $object != "..") {
                    if (!is_writable_r($dir."/".$object)) return false;
                    else continue;
                }
            }    
            return true;    
        }else{
            return false;
        }
        
    }else if(file_exists($dir)){
        return (is_writable($dir));
        
    }
}

?>

It appears that is_writable() does not check full permissions of a file to determine whether the current user can write to it.  For example, with Apache running as user 'www', and a member of the group 'wheel', is_writable() returns false on a file like

-rwxrwxr-x           root         wheel          /etc/some.file

Regarding you might recognize your files on your web contructed by your PHP-scripts are grouped as NOBODY you can avoid this problem by setting up an FTP-Connection ("ftp_connect", "ftp_raw", etc.) and use methods like "ftp_fput" to create these [instead of giving out rights so you can use the usual "unsecure" way]. This will give the files created not the GROUP NOBODY - it will give out the GROUP your FTP-Connection via your FTP-Program uses, too.

Furthermore you might want to hash the password for the FTP-Connection - then check out:
http://dev.mysql.com/doc/mysql/en/Password_hashing.html

This file_write() function will give $filename the write permission before writing $content to it.

Note that many servers do not allow file permissions to be changed by the PHP user.

<?php
    function file_write($filename, &$content) { 
        if (!is_writable($filename)) {
            if (!chmod($filename, 0666)) {
                 echo "Cannot change the mode of file ($filename)";
                 exit;
            };
        }
        if (!$fp = @fopen($filename, "w")) {
            echo "Cannot open file ($filename)";
            exit;
        }
        if (fwrite($fp, $content) === FALSE) {
            echo "Cannot write to file ($filename)";
            exit;
        } 
        if (!fclose($fp)) {
            echo "Cannot close file ($filename)";
            exit;
        }
    } 
?>

The results of this function seems to be not cached :
Tested on linux and windows

<?php
chmod($s_pathFichier, 0400);
echo'<pre>';var_dump(is_writable($s_pathFichier));echo'</pre>';
chmod($s_pathFichier, 04600);
echo'<pre>';var_dump(is_writable($s_pathFichier));echo'</pre>';
exit;
?>

This function returns always false on windows, when you check an network drive.

See PHP Bug https://bugs.php.net/bug.php?id=68926
See https://stackoverflow.com/q/54904676

I've encountered an unexpected issue: even though the directory allows writing files, it keeps returning false.

```
$a = 'C:/Users/*/OneDrive/work/www/';
$b = "{$a}admin/";
var_dump($a);
var_dump(is_writable($a));
var_dump($b);
var_dump(is_dir($b));
var_dump(is_writable($b));
var_dump(file_put_contents($b.'test.txt','hello'));
```

- string 'C:/Users/*/OneDrive/work/www/' (length=33)
- boolean true
- string 'C:/Users/*/OneDrive/work/www/admin/' (length=39)
- boolean true
- boolean false
- int 5