$filesystem = new \Symfony\Component\Filesystem\Filesystem();
$filesystem->makePathRelative(
    endPath: '/var/www/html/project/foo/bar/tmp/test.txt',
    startPath: '/var/www/html/project'
); // output: "foo/bar/tmp/test.txt/"

<?php

require __DIR__ . '/vendor/autoload.php';

$filesystem = new \Symfony\Component\Filesystem\Filesystem();

// Init situation
$projectDir = __DIR__;
$tmpFolderPath = "{$projectDir}/foo/bar/tmp";
$tmpFilePath = "{$tmpFolderPath}/test.txt";

$filesystem->mkdir($tmpFolderPath, 0700);
$filesystem->touch($tmpFilePath);

// Output: foo/bar/tmp/test.txt/
// Expected output: foo/bar/tmp/test.txt
$relativePath = $filesystem->makePathRelative($tmpFilePath, $projectDir);

// Debug info
dump([
    'projectDir' => $projectDir, // ✔ "/var/www/html/project"
    'tmpFolderPath' => $tmpFolderPath, // ✔ "/var/www/html/project/foo/bar/tmp"
    'tmpFilePath' => $tmpFilePath, // ✔ "/var/www/html/project/foo/bar/tmp/test.txt"
    'relativePath' => $relativePath, // ✘ "foo/bar/tmp/test.txt/" – expected: "foo/bar/tmp/test.txt"
    'tmpFolderPath.isFile' => file_exists($tmpFolderPath), // ✔ true
    'tmpFolderPath.isFolder' => is_dir($tmpFolderPath), // ✔ true
    'tmpFilePath.isFile' => file_exists($tmpFilePath), // ✔ true
    'tmpFilePath.isFolder' => is_dir($tmpFilePath), // ✔ false
]);