PHP is_writable Function

last modified April 3, 2025

The PHP is_writable function checks whether a file or directory is writable. It's essential for file operations to avoid permission errors.

Basic Definition

The is_writable function checks if the specified filename exists and is writable. It returns true if the file exists and is writable.

Syntax: is_writable(string $filename): bool. The function checks the effective user's permissions, not just the file mode.

Basic is_writable Example

This shows the simplest usage of is_writable to check a file.

basic_is_writable.php

<?php

declare(strict_types=1);

$file = "data.txt";

if (is_writable($file)) {
    echo "The file is writable";
} else {
    echo "The file is not writable";
}

This checks if "data.txt" is writable by the current user. The result depends on both file permissions and the user running the script.

Checking Directory Writability

is_writable can also check if a directory is writable.

directory_check.php

<?php

declare(strict_types=1);

$dir = "/var/www/uploads";

if (is_writable($dir)) {
    echo "The directory is writable";
} else {
    echo "The directory is not writable";
}

This verifies if the web server can write to the uploads directory. Directory writability is needed for creating or modifying files within it.

Checking Before File Write

A practical example checking writability before attempting to write.

before_write.php

<?php

declare(strict_types=1);

$logFile = "application.log";

if (!is_writable($logFile)) {
    die("Cannot write to log file. Check permissions.");
}

file_put_contents($logFile, "New log entry\n", FILE_APPEND);
echo "Log entry written successfully";

This prevents errors by checking writability before writing. It's a good practice for robust file handling in applications.

Relative vs Absolute Paths

The function works with both relative and absolute file paths.

path_types.php

<?php

declare(strict_types=1);

$relative = "config/settings.ini";
$absolute = "/etc/php/8.2/php.ini";

var_dump(is_writable($relative));
var_dump(is_writable($absolute));

Both path types are valid. Relative paths are resolved relative to the current working directory of the script.

Checking Multiple Files

Example showing how to check writability for multiple files.

multiple_files.php

<?php

declare(strict_types=1);

$files = [
    "data.csv",
    "reports/output.log",
    "/tmp/php_uploads"
];

foreach ($files as $file) {
    $status = is_writable($file) ? "writable" : "not writable";
    echo "{$file} is {$status}\n";
}

This efficiently checks multiple files in one loop. The ternary operator makes the output concise and readable.

Edge Cases

is_writable has specific behaviors with special cases.

edge_cases.php

<?php

declare(strict_types=1);

// Non-existent file
var_dump(is_writable("nonexistent.txt"));

// Symbolic link
var_dump(is_writable("/var/www/html/symlink"));

// Windows share
var_dump(is_writable("\\\\server\\share\\file.txt"));

For non-existent files, it returns false. For symlinks, it checks the target. Windows network paths are also supported with proper permissions.

Best Practices

Error Handling: Combine with file_exists for complete checks.
Security: Validate paths before checking permissions.
Caching: Results may be cached; clearstatcache() if needed.
Portability: Works consistently across platforms.

Source

PHP is_writable Documentation

This tutorial covered the PHP is_writable function with practical examples showing its usage in different scenarios.

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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