How to open a file and read, write content in the file using php?

<?php
//File Read
$filename = "content.txt";
$fget = fopen($filename, 'r');
$theData = fread($fget, filesize($filename));
fclose($fget);
echo $filecontent;

//File Write
$filename = "content.txt";
$fget = fopen($filename, 'w') or die("can't open file");
$filecontent = "Test content1\n";
fwrite($fget, $stringData);
$filecontent = "Test content2\n";
fwrite($fget, $filecontent);
fclose($fget);

?>