Custom task in grumphp-shim

composer.json

{
    "scripts": {
        "post-install-cmd": [
            "php -d phar.readonly=Off ./InstallTask.php"
        ]
    }
}

InstallTask.php

#!/usr/bin/env php
<?php

$phar = new Phar('vendor/phpro/grumphp-shim/grumphp.phar');
$phar->addFile('MyCustom.php', 'src/Task/MyCustom.php');

$newAutoloaderContent = <<<FILE
'GrumPHP\\Task\\MyCustom' => __DIR__ . '/../..' . '/src/Task/MyCustom.php',
FILE;

$autoloader = $phar->offsetGet('vendor/composer/autoload_static.php');
$file = $autoloader->openFile('rb+');
$temp = fopen('php://temp', 'rwb+');
while (!$file->eof()) {
    fwrite($temp, $file->fread(1024));
}
$file->seek(392); // Somewhere in the $classMap
$currentLine = $file->fgets();
if (stripos($currentLine, $newAutoloaderContent) !== false) {
    echo 'Already added.';
    return;
}
$file->seek(392); // Reset pointer after read
fseek($temp, $file->ftell());
fwrite($temp, $newAutoloaderContent);
while (!$file->eof()) {
    fwrite($temp, $file->fread(1024));
}
rewind($temp);
$file = null; // Close the file to allow overwriting
$contents = stream_get_contents($temp);
// If you just write to the $file stream it will corrupt the phar
$phar->addFromString('vendor/composer/autoload_static.php', $contents);