也可以使用此包: https://github.com/symfony/mime

function check_is_img($file) {
    $maps = [
        'jpeg' => ['image/jpeg', 'image/pjpeg'],
        'jpg' => ['image/jpeg', 'image/pjpeg'],
        'png' => ['image/png', 'image/apng', 'image/vnd.mozilla.apng'],
        'gif' => ['image/gif'],
    ];

    // get mime type.
    $image_mime = mime_content_type($file);

    // get ext.
    $ext = pathinfo($file, PATHINFO_EXTENSION);

    if (!isset($maps[$ext])) {
        return false;
    }

    $file_mine = $maps[$ext];
    return in_array($image_mime, $file_mine);
}

$file = './test_php.png';
var_dump(check_is_img($file));