PHP: retrieve a file extension by its contents

file_extension_by_contents.md

Using Symfony's MimeTypes

use Symfony\Component\Mime\MimeTypes;

function getFileExtension(string $fileContent): ?string
{
    $mimeType = (new \finfo(FILEINFO_MIME_TYPE))->buffer($fileContent);

    $mimeTypes = new MimeTypes();

    $extensions = $mimeTypes->getExtensions($mimeType);

    return $extensions[0] ?? null;
}

Using a plain/pure PHP

function getImageExtension(string $fileContent): ?string
{
    $mimeType = (new \finfo(FILEINFO_MIME_TYPE))->buffer($fileContent);

    // a custom mapping
    return match ($mimeType) {
        'image/gif' => 'gif',
        'image/png' => 'png',
        'image/jpeg' => 'jpg',
        default => null,
    };
}