/**
* 图片链接转换为 base64 文件流
* @param $imgUrl
* @return string
*/
function img_url_to_base64($imgUrl)
{
     $imageInfo = getimagesize($imgUrl);
      return 'data:' . $imageInfo['mime'] . ';base64,' . chunk_split(base64_encode(file_get_contents($imgUrl)));
}

展示：

{
    "code": 0,
    "message": "操作成功",
    "data": {
        "img_base64": "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAcUAAAHFAQAAAACrut8pAAAAAmJLR0QAAd2KE6QAAAAJcEhZcwAA\r\nAEgAAABIAEbJaz4AAAAJdnBBZwAAAcUAAAHFAEXPcIYAAAOeSURBVHhe7ZxNbiIxEIU7QgrL3CBc\r\ngQNE4lrsmqNxlByhlywQHr/6cdOI/pExE7m63iYU9hdNntqvbCdMEzJ1aR7fWSon5+TknCLZNUkf\r\nj8NDXfuZTVuYvGzCrem24Yr6sjl/x+J0cLJZHTmmkmQrI+dZcievurLk75c6dPrh73Sm+dEuJ1dF\r\nTqgoSYtgu4jkJVWUjD4E/Otv6FQoIBScCU6uhpyUIVJNiUsQXfD3SxfvrENO2iInZYgkU44bmghT\r\n+n3fz4eTayInVZSUkSUkqyiJTIgOJVOoJUpAOLkmclIlyaRZsldBkhxqw3knpgwKJ1dETqocmaly\r\nJLqgvh50QbLrXk7aJperOpJWvjS+gNusGAPxON/hypY2gXcB0U9z0h6ZqepI/OhQf4F12egwO/Qd\r\nR3gTeOxvdp00R2aqShK7uz4TjuwQmXIn6o8cEMFJyCCZoQrJ+KNfZOUf2CHygR3SEcoEMTJlgpO2\r\nyCxVR17ZB25vB347OkR2RVM0IOCd2gU5aY7MVHUkOSSmBAQEm0IrnwNCYwCZQJJMcNIWmanqSHUI\r\nPgTcZsEUdMEAu4YjLWZ2FB1OmiMzVSXJMUAihwJ8IO+iKewQjbSYQYWTNskMVUjyhm6npiQhBvgV\r\njdzt+66DM5KTVsgsVUfCFMTArdHDTYtNoJhyN6IFbwKdNEdmqkrySa8LOM9wJsAUMjJFh5NWyQxV\r\nSNJi5/bGb6Ze148gBhAdZCSiw0mDZJaqJDkGuL1RQOxQSwzwTM0EsuvgpFEyQ9WRWPnJBwQECjQ+\r\n+EAxgMYXixgQLWZwS3TSHJmp6kgsdo4B9YFMocZHpvQtMX7hkYOTBslMVUf2DoVh44MPVCAgyKE4\r\nLQzSxElLZKb+B9nJh0jokB1eJMUhbXzaErVgbHvaC/nZPKaJk0bIheqELPL0LZR8rPANpKx8bnxQ\r\n2vftE9k9vb910hY5rrlnaFxPSF0EzfTnBTt8dHz3HlIdYh+QFskhITu9v3XSNDmm+WdoTE/JVorz\r\nDIkvbyCvbEqUOARRf9wL+SneOWmbHJdMG32GxvWcXPL/fuiSKk/CobjvoxigCbELAtsmEvs+J+2T\r\n41Jy7BkaVz75qHJk71Dg2yy1S489qT86aZtcrrpJOIQYuPHKV1P4OH8/4qRxcrleJaX4M5JOOuyD\r\nBAQV7FDLxx4n10Au1Ytk0t+QdJt1TH+GRJkQ1dEvKQZ2OWmbXK6XyEw5OScn5/QC+Q80ME27gJ1j\r\n9gAAACV0RVh0ZGF0ZTpjcmVhdGUAMjAyMC0wNi0xNlQxNDo1MjowOCswODowMJs4XlEAAAAldEVY\r\ndGRhdGU6bW9kaWZ5ADIwMjAtMDYtMTZUMTQ6NTI6MDgrMDg6MDDqZebtAAAAAElFTkSuQmCC\r\n"
    }
}