php解析base64数据成图片

$base64 = "/9j/4AAQSkZJRgABAQEAkACQAAD/4QCMRXhpZgAATU0AKgAAAAgABQESAAMAAAABAAEAAAEaAAUAAAABAAAASgEbAAUAAAABAAAAUgEoAAMAAAABAAIAAIdpAAQAAAABAAAAWgAAAAAAAACQAAAAAQAAAJAAAAABAAOgAQADAAAAAQABAACgAgAEAAAAAQAAAHKgAwAEAAAAAQAAAHIAAAAA/9sAQwAfFRcbFxMfGxkbIyEfJS9OMi8rKy9fREg4TnBjdnRuY21rfIyyl3yEqYZrbZvTnam4vsjKyHiV2+rZwumyxMjA/9sAQwEhIyMvKS9bMjJbwIBtgMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDAwMDA/8AAEQgAcgByAwEiAAIRAQMRAf/EAB8AAAEFAQEBAQEBAAAAAAAAAAABAgMEBQYHCAkKC//EALUQAAIBAwMCBAMFBQQEAAABfQECAwAEEQUSITFBBhNRYQcicRQygZGhCCNCscEVUtHwJDNicoIJChYXGBkaJSYnKCkqNDU2Nzg5OkNERUZHSElKU1RVVldYWVpjZGVmZ2hpanN0dXZ3eHl6g4SFhoeIiYqSk5SVlpeYmZqio6Slpqeoqaqys7S1tre4ubrCw8TFxsfIycrS09TV1tfY2drh4uPk5ebn6Onq8fLz9PX29/j5+v/EAB8BAAMBAQEBAQEBAQEAAAAAAAABAgMEBQYHCAkKC//EALURAAIBAgQEAwQHBQQEAAECdwABAgMRBAUhMQYSQVEHYXETIjKBCBRCkaGxwQkjM1LwFWJy0QoWJDThJfEXGBkaJicoKSo1Njc4OTpDREVGR0hJSlNUVVZXWFlaY2RlZmdoaWpzdHV2d3h5eoKDhIWGh4iJipKTlJWWl5iZmqKjpKWmp6ipqrKztLW2t7i5usLDxMXGx8jJytLT1NXW19jZ2uLj5OXm5+jp6vLz9PX29/j5+v/aAAwDAQACEQMRAD8A6GiiigAooqF5scL+dAEpIHU4phmQepquSWOSc0UwJvPH939aPP8A9n9ahooAsCZT1yKeCG6HNVKASDkHFAFyioEm7N+dTAgjIpALRRRQAUUUUAFFFQzv/CPxoAbLJuOB0/nUdFFMAoopyIXPH50ANoqTES9SWPtR+6b1WgCOinPGU56j1ptABTo5Ch9vSm0UAWwQRkdKWq8L4O09DVikAUUUUAIx2qT6VUJycmp5zhQPWoKYBRRRQAdalkOxQi/jUaffX606b/WGgBlFFFAEkTZ+RuhpjDaxHpQn31+tOm/1hoAZRRRQAVajbcgPeqtS255I/GgCeiiikBBcfeA9qiqS4++PpUdMAooooAKlceYoZeo6ioqkjRh82do96AI6KmZoieRk+1N3xr91Mn3oAI12je3QdKjJyST3pXcueaSgAooooAKfD/rBTKfD/rBQBZooopAQ3A6GoasyruQiq1MAoopVG5gPWgB6KFXe34CmO5c5NOmPzbR0FMoAKKKKACpNoePKjkdRUdOiba49+KAG0U6RdrkU2gAqSAfOT6Co6sQLhM+tAElFFFIAqtKm1vY1ZprqHXBoAq06MhXBPSkZSpwaSmArHLE+ppKKKACiiigAooooAfKwZsj0plFABJwOtADkXe2KtdKZGmxffvT6QBRRRQAUUUUANdA4waruhQ89PWrVFAFOip2hU9OKYYXHTBpgR0U7Y/8AdNJsb+6fyoASiniFz2xUiwAfeOaAIVUscAVYjjCD1PrTgABgDFLSAKKKKACiiigAooooAKKKKACiiigAooooAKKKKACiiigAooooAKKKKAP/2Q==";
$img = base64_decode($base64);
$a = file_put_contents('./test.jpg', $img);//返回的是字节数
print_r($a);