What are the best practices for checking file permissions and executability in PHP?

When working with files in PHP, it is important to check file permissions to ensure that the file is accessible and executable. This can be done using the `file_exists()` function to check if the file exists, and the `is_readable()` and `is_executable()` functions to check if the file is readable and executable, respectively.

$file = 'path/to/file.txt';

if (file_exists($file)) {
    if (is_readable($file)) {
        echo 'File is readable.';
    } else {
        echo 'File is not readable.';
    }

    if (is_executable($file)) {
        echo 'File is executable.';
    } else {
        echo 'File is not executable.';
    }
} else {
    echo 'File does not exist.';
}