Loop through text file, amend & write new file PHP - php

Question

I have a file called test.txt. It has multiple lines of text in it like thus:
Test Data:
Tester 2 Data:
Tests 3 Data:
I would like to have a PHP script that opens this file, strips ALL of the text before the word Data: on each row and outputs the result:
Data:
Data:
Data:
My PHP so far:
$myfile = fopen("test.txt", "r") or die("Unable to open file!");
$data = fread($myfile,filesize("test.txt"));
// foreach line do this
$line = strstr($data,"Data:");
//append $line to newtest.txt
// endforeach
fclose($myfile);

Answer 1

You can use file() to open and loop over a file line by line.
As you are removing everything before Data:, based on your test data supplied (which is all I have to go on), we only need to know the number of lines. So, we can use count() to get that information.
Then construct the new data as a variable, and finally write that variable to a (new) file using file_put_contents().
Using trim() will remove the last extra line return.
$raw = file("./test.txt");
$lineCount = count($raw);
$newFile = null;
do {
$newFile .= "Data:\r\n";
} while(--$lineCount > 0);
file_put_contents('./test-new.txt',trim($newFile));
Edit:
As Don't Panic says in the comment below, you could use str_repeat() to even remove the do while loop.
Here is that version with the count() moved in-line as well:
$raw = file("./test.txt");
$newFile = str_repeat("Data:\r\n",count($raw));
file_put_contents('./test-new.txt',trim($newFile));