PHP Script wipes strings completely - how to fix? - php

Question

I have made a script which saves a few lines of PHP to a .php file. All my scripts work perfectly fine, but just this page is starting to get annoying.
Explaination of what is happening in the GIF:
1: I change the settings - the last 2 settings need to appear when you enable the 2nd setting called "Custom style". That works fine and all.
2: So you enable it and for some reason it completely wipes the other 2 settings (the "primarycolor" and "adminbg").
How can this happen? What am I doing wrong? My script is below if you want to try it out yourself.
<?php
if (isset($_POST["submit"])) {
$string = '<?php
$customoptions = '. $_POST["customoptions"] .';
$primarycolor = "'. $_POST["primarycolor"] .'";
$adminbg = "'. $_POST["adminbg"] .'";
?>';
$fp = fopen("includes/userstyle.php", "w");
fwrite($fp, $string);
fclose($fp);
}
include("includes/userstyle.php");
?>
<form action="" name="customopt" method="post">
<table>
<tr>
<td>Panel language</td>
<td>
<select onchange="this.options[this.selectedIndex].value && (window.location = this.options[this.selectedIndex].value);">
<option><?php echo $lang['chooselanguage']; ?></option>
<option value="dashboard.php?lang=en">English</option>
<option value="dashboard.php?lang=nl">Dutch</option>
</select>
</td>
</tr>
<tr>
<td>Custom Style</td>
<td><select name="customoptions" id="customoptions"><option value="true" <?php if($customoptions == true){ echo 'selected'; }; ?>><?php echo $lang['enabled']; ?></option><option value="false" <?php if($customoptions == false){ echo 'selected'; }; ?>><?php echo $lang['disabled']; ?></option></select></td>
</tr>
<?php if($customoptions) { ?>
<tr>
<td>Primary Color</td>
<td><input name="primarycolor" type="text" id="primarycolor" value="<?php echo $primarycolor; ?>"></td>
</tr>
<tr>
<td>Background Color</td>
<td><input name="adminbg" type="text" id="adminbg" value="<?php echo $adminbg; ?>"></td>
</tr>
<?php } ?>
</table>
<input type="submit" name="submit" value="<?php echo $lang['ok']; ?>">
</form>
EDIT: userstyle.php
<?php
$customoptions = true;
$primarycolor = "555";
$adminbg = "fff";
?>

Answer 1

When you post your form for the second time, you post empty values, and they are saved to file, as expected.
You should add something like
if (isset($_POST["submit"]) && !empty($_POST['primarycolor']) && !empty($_POST['adminbg'])) {
// ...
But actually there can be another validation rules.
And as others noticed in comments — this is, even in educational purposes, very stupid idea to save user data into php file and then execute that file. The simplest alternative — save settings to a json file with json_encode, then decode, and don't forget to html-escape them with at least htmlspecialchars.