A collection of function for git hash checking in PHP. Pure PHP, no command exec() required.

git_helper.php

<?php
/**
 * Git Helper
 *
 * This file contains set of functions that returns git's metadata.
 * It doesn't perform git operation; read then pharse only.
 *
 * LIMITATION: this function only read local object (branch, tag).
 * You have to fetch remote object first (eg. `git fetch --all`).
 * Best use on shared web hosting and enviorment with exploitable
 * funtions disabled: shell_exec(), exec(), eval(), system(), etc.
 *
 * $git_root param is relative to frontend file (index.php).
 * eg. "is_git('.');" will check the dir of index.php.
 *
 * Set $git_root param to NULL, it will use APPPATH.
 *
 * @author Indra Kurniawan <indekaw@teknik.io>
 * @copyright 2019 Indra Kurniawan
 * @version 1.0.0
 */

// This's a helper of designed for CodeIgniter. If you are having problem
// accesing it, you should comment out or remove a line bellow.
defined('BASEPATH') or exit('No direct script access allowed');

/**
 * Determinte is application is run undergit.
 *
 * @param	string $git_root
 * @param	boolean $check_parent
 * @return	boolean|string FALSE or path to path to HEAD file
 */
function is_git($git_root = NULL, $check_parent = FALSE)
{
	if (is_null($git_root)) {
		if (defined(APPPATH)) {
			// This's a helper of CodeIgniter, so...
			$git_root = APPPATH;
		} else {
			$git_root = '.';
		}
	}

	if (file_exists($head_path = $git_root . '/.git/HEAD')) {
		return $head_path;
	}

	if ($check_parent && (file_exists($head_path = $git_root . '/../.git/HEAD'))) {
		return $head_path;
	}

	return FALSE;
}

/**
 * Return referenced path of HEAD
 *
 * @param	string $git_root
 * @param	boolean $check_parent
 * @return	boolean|string FALSE or ref path string
 */
function git_ref($git_root = NULL, $check_parent = FALSE)
{
	// Is valid?
	if (($head_path = is_git($git_root, $check_parent)) === FALSE) {
		return FALSE;
	}

	// Does HEAD contain ref?
	if (strpos($head_content = @file_get_contents($head_path), 'ref:') === 0) {
		$head_ref = trim(str_replace('ref:', '', trim($head_content)));
		return sprintf(dirname($head_path).'/%s', $head_ref);
	}

	return FALSE;
}

/**
 * Check whatever the HEAD file refer to a branch or not.
 * This implies git_ref();
 *
 * @param	string $git_root
 * @param	boolean $check_parent
 * @return	boolean|string FALSE or path to branch's name
 */
function git_branch($git_root = NULL, $check_parent = FALSE)
{
	// Validate git then return HEAD's ref.
	if (($ref_head = git_ref($git_root, $check_parent)) === FALSE) {
		return FALSE;
	}

	// Does ref contain branch path?
	if (strpos($ref_head, '.git/refs/heads/') !== FALSE) {
		return $ref_head;
	}

	return FALSE;
}

/**
 * Check whatever the HEAD file refer to a tag or not.
 * This implies git_ref();
 *
 * @param	string $git_root
 * @param	boolean $check_parent
 * @return	boolean|string FALSE or path to tag's name
 */
function git_tag($git_root = NULL, $check_parent = FALSE)
{
	// Validate git then return HEAD's ref.
	if (($ref_head = git_ref($git_root, $check_parent)) === FALSE) {
		return FALSE;
	}

	// Does ref contain tag path?
	if (strpos($ref_head, '.git/refs/tags/') !== FALSE) {
		return $ref_head;
	}

	return FALSE;
}

/**
 * Return hash of HEAD file
 *
 * @param string $git_root
 * @param boolean $check_parent
 * @return	boolean|string FALSE or HEAD's hash
 */
function git_hash($git_root = NULL, $check_parent = FALSE)
{
	// Is head a ref?
	if ($head_ref = git_ref($git_root, $check_parent)) {
		return trim(@file_get_contents($head_ref));
	}

	// Assume HEAD contains the hash.
	if ($head_path = is_git($git_root, $check_parent)) {
		return trim(@file_get_contents($head_path));
	}

	// Maybe it isn't a git.
	return FALSE;
}

/**
 * Return the refferenced branch (or tag) of HEAD's hash.
 * Best use for `git checkout <branch>` instead of ugly hash.
 *
 * @param string $git_root
 * @param boolean $check_parent
 * @return	boolean|string FALSE or head's ref (branch or tag) name
 */
function git_head($git_root = NULL, $check_parent = FALSE)
{
	// Is head a ref?
	if ($head_ref = git_ref($git_root, $check_parent)) {
		return basename($head_ref);
	}

	// Lookup for refs by HEAD's hash.
	if (!$head_path = is_git($git_root, $check_parent)) {
		return FALSE;
	}

	// Let's see which branch or tags has this hash (no pun needed)/
	$object_list = array();
	$object_list = glob(dirname($head_path) . '/refs/{heads,tags}/*', GLOB_BRACE);

	// A repo basically has a branch, if isn't then it's an empty repo (has no commit)
	if (empty($object_list)) {
		return FALSE;
	}

	// Looping throught array of files within read its contents isn't that slow.
	// They are branches and tags files, a popular git repo usually has around 60 of them.
	$hash = trim(@file_get_contents(trim($head_path)));
	foreach ($object_list as $file) {
		if ((trim(@file_get_contents(trim($file)))) === $hash) {
			// return only first found
			return basename($file);
		}
	}

	// Maybe it isn't a git or hash has no refs
	return FALSE;
}

procedural_example.php

<?php
/**
 * Example file
 * 
 * Run this example script by command-line interface (no web-server required):
 * $ cd path/to/this/file/
 * $ php ./procedural_example.php
 */

// Define BASEPATH (required, for to prevent direct access).
define('BASEPATH', '');
// Load the helper
require_once 'git_helper.php';

// Set repo is on curent dir relatively to this file (procedural_example.php)
// You might set this to magic constants __DIR__, etc.
$path = '.';
// No need to check the parent
$check_parent = FALSE;

var_dump($path);
var_dump($check_parent);
echo '=================' . PHP_EOL;
echo 'is_get: ';
var_dump(is_git($path, $check_parent));
echo 'head\'s content: ';
var_dump(@file_get_contents(is_git($path, $check_parent)));
echo '=================' . PHP_EOL;
echo 'git_branch: ';
var_dump(git_branch($path, $check_parent));
echo 'git_tag: ';
var_dump(git_tag($path, $check_parent));
echo 'git_hash: ';
var_dump(git_hash($path, $check_parent));
echo 'git_ref: ';
var_dump(git_ref($path, $check_parent));

// EOF