Read .txt with php and write to the same file - php

Question

I'm working on what seems to be a simple php reading and writing to a txt file. I'm able to read the file but not write. I've tried variations of the following but to no avail. I'm trying to read the txt file that will have a 0 or a 1 in it and switch it. But for some reason I can only read. How can I make this code write to the file? Note I do have write permissions to the txt file.
Thx
Rich
<?php
$status = readfile("0.txt");
echo $status;
$file_handle = fopen("../light_switches/0.txt", "w");
if($status = 1){
$file_contents = "0";
fwrite($file_handle, $file_contents);
}
else if($status = 0){
$file_contents = "1";
fwrite($file_handle, $file_contents);
}
fclose($file_handle);
?>

Answer 1

readfile returns the number of bytes read from the file, but not the file content.
You could use file_get_contents to get the file content, and use file_put_contents to write:
$path = "../light_switches/0.txt";
file_put_contents($path, file_get_contents($path) === '0' ? '1' : '0');

Answer 2

Use file_put_contents(). It is the same as calling fopen(), fwrite(), and fclose(). Use the flag FILE_APPEND
file_put_contents($filepath, $content, FILE_APPEND);
PHP Documentation
EDIT
I have reworked this code. If all you are doing is trying to switch the 0 and 1 of the txt file, this code is working. Simply just replace the file path with yours.
<?php
$file_path = '0.txt';
if (!file_exists($file_path)) {
echo 'File does not exist';
exit;
}
$currentContent = file_get_contents($file_path);
echo $currentContent;
if ($currentContent == '0') {
$newContent = '1';
}
elseif ($currentContent == '1') {
$newContent = '0';
}
else {
//Start the file fresh if it found extra lines for example
$newContent = '0';
}
file_put_contents($file_path, $newContent);
?>