Take content from one text file and put it in another with PHP?

i need help with this one , is there any ways when i amend file1.txt to send the old information to file2.txt and so on. I have 4 files like this and i want to swap the content inside them when file1.txt is changed .

Like this when content in first one changed send old content in the chain file1.txt --> file2.txt --> file3.txt--> file4.txt

thats my php file

$myfile = fopen("file1.txt", "w") or die("Unable to open file!");
fwrite($myfile, "Today we had 11 people");
fclose($myfile);

my idea is to send This content of strings "Today we had 11 people" to second file , when content in file1.txt is changed to lets say "Today we had 23 people"

Easy/pragmatic way to do that would be copy the files in the chain.

function copyFiles(string $newLine): void
{

    exec("cp path/to/file3.txt path/to/file4.txt");    
    exec("cp path/to/file2.txt path/to/file3.txt");
    exec("cp path/to/file1.txt path/to/file2.txt");

    exec("echo $newLine >> path/to/file1.txt");
}

Note: For this approach, you may need to talk to your host to see if it is allowed at all. not every host allows cp commands via php exec.

suggestion from @Will

You can also use the rename() function instead of exec() for this. rename("/test/file1.txt","/home/docs/my_file.txt");

Thank you to @Maik Lowrey for the answer , how he noted cp is not allowed on my host. Instead of that i used the idea of @Will and worked perfectly ! THANK YOU BOTH :)

function copyFiles(string $newLine): void
{

    rename( "file3.txt", "file4.txt");    
    rename("file2.txt", "file3.txt");
    rename("file1.txt", "file2.txt");

    exec("echo $newLine >> file1.txt");
}