Modifying a single text file in a ZIP file, in PHP

Have you taken a look at PHP5's ZipArchive functions?

Basically, you can use ZipArchive::Open() to open the zip, then ZipArchive::getFromName() to read the file into memory. Then, modify it, use ZipArchive::deleteName() to remove the old file, use ZipArchive::AddFromString() to write the new contents back to the zip, and ZipArchive::close():

$zip = new ZipArchive;
$fileToModify = 'myfile.txt';
if ($zip->open('test1.zip') === TRUE) {
    //Read contents into memory
    $oldContents = $zip->getFromName($fileToModify);
    //Modify contents:
    $newContents = str_replace('key', $_GET['param'], $oldContents)
    //Delete the old...
    $zip->deleteName($fileToModify)
    //Write the new...
    $zip->addFromString($fileToModify, $newContents);
    //And write back to the filesystem.
    $zip->close();
    echo 'ok';
} else {
    echo 'failed';
}

Note ZipArchive was introduced in PHP 5.2.0 (but, ZipArchive is also available as a PECL package).

In PHP 8 you can use ZipArchive::replaceFile

As demonstrated by this example from the docs:

<?php
$zip = new ZipArchive;
if ($zip->open('test.zip') === TRUE) {
    $zip->replaceFile('/path/to/index.txt', 1);
    $zip->close();
    echo 'ok';
} else {
    echo 'failed';
}
?>