chmod()

Understanding PHP chmod() Function

chmod() changes the mode (the permission bits) of a file or directory on a Unix-like filesystem. The mode controls who can do what with the file: read its contents, write to it, or execute it. Getting permissions right matters for security — too permissive and anyone can read or overwrite sensitive data; too strict and your web server can't read the files it needs to serve.

This page covers the syntax, how to read and write the octal mode, the most common permission values you'll actually use, the return value, and the gotchas that trip people up (the leading zero, umask, and Windows).

Syntax

chmod(string $filename, int $permissions): bool

It returns true on success and false on failure.

How the octal mode works

The mode is a three-digit octal number. Each digit describes one class of user:

Each digit is the sum of the permissions you grant to that class:

Add them up: read + write = 4 + 2 = 6, and read + execute = 4 + 1 = 5. So 0750 means owner: read/write/execute (7), group: read/execute (5), others: nothing (0).

Common permission values

Examples

Set permissions on a single file

The mode 0644 lets the owner read and write, while the group and everyone else can only read:

<?php
if (chmod("example.txt", 0644)) {
    echo "Permissions updated.";
} else {
    echo "Failed to change permissions.";
}

Make a script executable

To run a PHP script directly (e.g. a CLI cron job), the owner needs execute permission:

<?php
chmod("backup.php", 0744); // owner: rwx, group & others: read only

Always check the return value

chmod() fails silently by returning false — for example when the script doesn't own the file. Don't assume it worked:

<?php
$file = "config.ini";
if (!chmod($file, 0600)) {
    error_log("Could not secure {$file}");
}

Gotchas

• Ownership matters. A process can only chmod() a file it owns (or as the superuser). On shared hosting the web server user often isn't the owner, so the call returns false.
• umask does not affect chmod(). Unlike mkdir() and file creation, chmod() sets the mode you ask for verbatim. The umask only masks newly created files — see umask().
• Windows ignores most bits. NTFS has no Unix permission model, so on Windows only the read-only bit is meaningfully affected.
• Check before you change. Use is_writable() to test access, and fileperms() to read the current mode before overwriting it.

Related functions

• fileperms() — read a file's current permission bits.
• is_writable() — test whether the script can write to a path.
• umask() — set the default mask applied to newly created files.
• mkdir() — create a directory with a given mode.

Conclusion

chmod() is your tool for tightening or loosening access to files and directories from PHP. Remember the three rules that prevent most bugs: write the mode in octal with a leading zero, pick the least permission that still works (0644 for files, 0755 for directories), and check the return value because the function fails quietly when your script doesn't own the target.

Practice