PHP Replace string in text file using - php

Question

I have a file with usernames and displaynames store in it like this:
testname=displayname<br>
testname2=displayname2<br>
etc=etc<br>
This list is done twice on the same file, same usernames and same displaynames. I need to replace them in both areas in the lists (needs to be replaced twice)
I am trying to create a form where uses can change their display name. I was trying to copy my code from a page where it looked up passwords for their accounts in a file, found it, and replaced it, however it doesn't seem to be working for this file.
The form for changing their names is simple, it has them enter in their member name (so I could use that to have it find their name in the list) and then uses what they input for a display name to change their display name in the file.
Form Page Code:
<center>Change Display Name:<p>
<form action="http://example.com/xxx/displaynamesave.php" class="form" method="post">
<input "membername" name="membername" /><p><input "displayname" name="displayname" /><p><input name="Submit" type="submit" /></form></p>/center>
and below is my php code for processing
<?
$fileurl = '/xxx/myfiles/sitename/xxx/memberfiletest';
$membername = $_POST['membername'];
$displayname = $_POST['displayname'];
$file = file($fileurl, FILE_IGNORE_NEW_LINES); // Get file as array of lines
foreach ($file AS $n=>$line)
if (substr($line, 0, 20) === '$membername') // Line starts with 'membername'
$file[$n] = '$membername = '.$displayname; // Replace displayname
file_put_contents($fileurl, implode("\n", $file)); // Put file back together
$success_page = 'http://example.com/thisplace/xxx/xxx/successredirector.html';
header('Location: '.$success_page);
?>
When I input the data and hit submit, it goes to my success page, however it doesn't make any changes in the proper file and I'm unsure how to tell what I'm missing.

Answer 1

First, your comparison shows the variable in single quotes ', which means PHP won't parse it and is in fact comparing everything to $membername, and not the value of the variable $membername. Change the comparison to:
foreach ($file as $n => $line) {
if (substr($line, 0, 20) === $membername) { // Line starts with 'membername'
$file[$n] = $membername . ' = ' . $displayname; // Replace displayname
}
}
Second, in the example of the contents of your file, the "username" portion are not all the same length ("testname", "etc"), but your comparison is checking against the first 20 characters of the line. If the format is indeed username=displayname, you would probably have better results splitting the line on the = (there are a couple ways to do this, of course), and comparing the first part. An example would be something like :
foreach ($file as $n => $line) {
$parts = explode('=', $line);
if ($parts[0] == $membername) {
$file[$n] = $membername . ' = ' . $displayname;
}
}

Answer 2

Use strpos
foreach ($file as $n => $line) {
if (strpos($line, $membername) === 0) { // Line starts with 'membername'
$file[$n] = $membername . ' = ' . $displayname; // Replace displayname
}
}

Answer 3

You're using variables inside a '. Try this:
foreach ($file as $n => $line) {
list($name, $dname) = explode('=', trim($line));
if ($name === $membername) { // Line starts with 'membername'
$file[$n] = $membername . ' = ' . $displayname; // Replace displayname
}
}