permission denied - php unlink

I have two files: b.php and test.txt

<?php 
$b = "test.txt";
unlink($b);
?>

and the error is: Warning: unlink(test.txt) [function.unlink]: Permission denied

why? b.php and test.txt is 777 and the same group/login

if I set 777 on the parent directory I can execute unlink but i have to set 777 and back to 755?

You (as in the process that runs b.php, either you through CLI or a webserver) need write access to the directory in which the files are located. You are updating the directory content, so access to the file is not enough.

Note that if you use the PHP chmod() function to set the mode of a file or folder to 777 you should use 0777 to make sure the number is correctly interpreted as an octal number.

You'll first require to close the file using fclose($handle); it's not deleting because the file is in use. So first close the file and then try.

in addition to all the answers that other friends have , if somebody who is looking this post is looking for a way to delete a "Folder" not a "file" , should take care that Folders must delete by php rmdir() function and if u want to delete a "Folder" by unlink() , u will encounter with a wrong Warning message that says "permission denied"

however u can make folders & files by mkdir() but the way u delete folders (rmdir()) is different from the way you delete files(unlink())

eventually as a fact:

in many programming languages, any permission related error may not directly means an actual permission issue

for example, if you want to readSync a file that doesn't exist with node fs module you will encounter a wrong EPERM error

// Path relative to where the php file is or absolute server path
chdir($FilePath); // Comment this out if you are on the same folder
chown($FileName,465); //Insert an Invalid UserId to set to Nobody Owner; for instance 465
$do = unlink($FileName);

if($do=="1"){ 
    echo "The file was deleted successfully."; 
} else { echo "There was an error trying to delete the file."; } 

Try this. Hope it helps.