PHP fileowner Function

last modified April 3, 2025

The PHP fileowner function retrieves the owner of a specified file. It returns the user ID of the file owner, which can be resolved to a username.

Basic Definition

The fileowner function returns the user ID of the owner of a file. It takes one parameter: the path to the file as a string.

Syntax: fileowner(string $filename): int|false. Returns the user ID on success or false on failure. The result is numeric on Unix systems.

Basic fileowner Example

This shows the simplest usage of fileowner to get a file owner ID.

basic_fileowner.php

<?php

declare(strict_types=1);

$filename = "/var/www/html/index.php";
$ownerId = fileowner($filename);

if ($ownerId !== false) {
    echo "File owner ID: " . $ownerId;
} else {
    echo "Could not get file owner";
}

This retrieves the numeric user ID of the file owner. The ID can be resolved to a username using posix_getpwuid on Unix-like systems.

Getting Owner Username

This example shows how to convert the owner ID to a readable username.

fileowner_username.php

<?php

declare(strict_types=1);

$filename = "/etc/passwd";
$ownerId = fileowner($filename);

if ($ownerId !== false) {
    $ownerInfo = posix_getpwuid($ownerId);
    echo "File owner: " . $ownerInfo['name'];
} else {
    echo "Could not get file owner";
}

Here we use posix_getpwuid to convert the numeric ID to a username. Note this function is only available on Unix-like systems with POSIX extension.

Checking File Ownership

This example checks if the current user owns a specific file.

check_ownership.php

<?php

declare(strict_types=1);

$filename = "test.txt";
$ownerId = fileowner($filename);
$currentUserId = posix_geteuid();

if ($ownerId !== false) {
    if ($ownerId === $currentUserId) {
        echo "You own this file";
    } else {
        echo "You don't own this file";
    }
} else {
    echo "Could not check file ownership";
}

We compare the file owner ID with the current effective user ID. This is useful for permission checks before performing sensitive file operations.

Error Handling

This demonstrates proper error handling when using fileowner.

error_handling.php

<?php

declare(strict_types=1);

$filename = "nonexistent.txt";
$ownerId = @fileowner($filename);

if ($ownerId === false) {
    $error = error_get_last();
    echo "Error: " . $error['message'];
} else {
    echo "File owner ID: " . $ownerId;
}

We use the @ operator to suppress warnings and check the return value explicitly. The error_get_last function retrieves the last occurred error.

Windows Compatibility

This shows fileowner behavior on Windows systems.

windows_compat.php

<?php

declare(strict_types=1);

$filename = "C:\\Windows\\win.ini";
$ownerId = fileowner($filename);

if ($ownerId !== false) {
    echo "File owner ID: " . $ownerId;
} else {
    echo "Could not get file owner (Windows may return 0)";
}

On Windows, fileowner typically returns 0 (SYSTEM) or may fail. Windows has different permission systems than Unix-like operating systems.

Best Practices

Error Handling: Always check for false return value.
Security: Validate file paths before use.
Windows: Be aware of different behavior on Windows.
Caching: Use clearstatcache if owner changes.
Permissions: Ensure script has permission to check owners.

Source

PHP fileowner Documentation

This tutorial covered the PHP fileowner function with practical examples showing its usage in different scenarios and operating systems.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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