In the context of PHP, what are the differences between using "r" and "w" file modes for reading and writing operations?

When opening files in PHP, using "r" mode allows for reading operations only, while "w" mode allows for writing operations only. If you want to read from a file, use "r" mode, and if you want to write to a file (and create it if it doesn't exist), use "w" mode. Be careful when using "w" mode, as it will overwrite the contents of an existing file.

// Reading from a file using "r" mode
$myfile = fopen("example.txt", "r") or die("Unable to open file!");
echo fread($myfile,filesize("example.txt"));
fclose($myfile);

// Writing to a file using "w" mode
$myfile = fopen("newfile.txt", "w") or die("Unable to open file!");
$txt = "Hello, world!";
fwrite($myfile, $txt);
fclose($myfile);