生成随机的字母数字字符串到.txt文件

I would like to create a random string of 25 alpha-numerical characters that when ran via a cron, would print the results to a key.txt file.

What would be the best way to do it? Thank you.

There are tons of tutorials out there, one possible solution (for a learning purpose, that is) would be:

$chars = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ';
// alternatively: implode('', array_merge(range(0,9),range('a', 'z'), range('A', 'Z')));
$length = 25;
$string = '';
for ($i = 0; $i < $length; $i++)
    $string .= $chars[rand(0, strlen($chars) - 1)];
// save it to a file
file_put_contents("key.txt", $string);

I can give you answers as long as the bible on how to generate random strings but you'll be reading them and saying to yourself: "Well, this sucks.". So I'll keep it short and to the point.

// Generate random string
$randomString = uniqid();

This way it will not have to be 25 characters long as the generated string is always unique and would be a perfect key or filename.

For more information check the PHP Manual on uniqid().

This is easy function to resolve your problem.
Pass your length variable to define length of out put.

function random_string( $length = 25 ) {
    $chars = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    $password = substr( str_shuffle( $chars ), 0, $length );
    return $password;
}

PS. it's not guarantee uniqueness of each output.