How to make fwrite() write in the middle of the text file - php

Question

$username = $_GET['username'];
$steps = $_GET['id'];
$myfile = fopen("save/" . $username . ".txt", "w") or die("Unable to open file!");
fwrite($myfile, $steps);
fclose($myfile);
echo $steps;
This is my code, The issue is that it is replacing the whole .txt file with just $steps while Test1|b7f2b0b64a3c60a367b40b579b06452d|Male|0|a|02/05/2016|1 is in the text file which therefore it ends up being just 32 if $steps = 32; in the text file, I want to replace the 0 with $steps = $_GET['id'];
hint I know its something like this
explode('|',$....)

Answer 1

It's because you are replacing the file's contents entirely, with $steps, which only contains the $_GET['id'].
Your explode idea can work towards this end, if you are sure that the steps will always be in the same place. Then it'd go somewhat like this.
$username = $_GET['username'];
$steps = $_GET['id'];
$myfile = fopen("save/" . $username . ".txt", "w") or die("Unable to open file!");
//explode on the |
$userData = explode('|', stream_get_contents($myfile));
//This should be the steps data. Replace it with the new value
$userData[3] = $steps;
//put the exploded string back together with the new steps value
fwrite($myfile, implode("|", $userData));
fclose($myfile);
fopen and fclose are quite intense operations for such small files, though. A possibly faster way (and easier, too!) is to do it like this:
$username = $_GET['username'];
$steps = $_GET['id'];
$myfile = file_get_contents("save/$username.txt");
//explode on the |
$userData = explode('|', $myfile);
//This should be the steps data. Replace it with the new value
$userData[3] = $steps;
//put the exploded string back together with the new steps value
file_put_contents("save/$username.txt", implode("|", $userData));
p.s. you can use $variables within the double " strings.

Answer 2

Note that you are using the w file mode, which will truncate the file on opening it. This means you will be losing data if you don't set it aside first. As per the documentation:
Open for writing only; place the file pointer at the beginning of the file and truncate the file to zero length. If the file does not exist, attempt to create it.
Using fgetcsv you could first read the data and then save the entire string back to the file:
$username = $_GET['username'];
$steps = $_GET['id'];
$myfile = fopen("save/" . $username . ".txt", "r+") or die("Unable to open file!");
// Get the current data from the file
$data = fgetcsv($myfile, 0, '|');
// Now replace the 4th column with our steps
$data[3] = $steps;
// Now truncate the file
ftruncate($myfile, 0);
// And save the new data back to the file
fwrite($myfile, implode('|', $data));
fclose($myfile);