Try
imagecreatefromstring — Create a new image from the image stream in the string
Example from PHP manual:
$data = 'iVBORw0KGgoAAAANSUhEUgAAABwAAAASCAMAAAB/2U7WAAAABl'
. 'BMVEUAAAD///+l2Z/dAAAASUlEQVR4XqWQUQoAIAxC2/0vXZDr'
. 'EX4IJTRkb7lobNUStXsB0jIXIAMSsQnWlsV+wULF4Avk9fLq2r'
. '8a5HSE35Q3eO2XP1A1wQkZSgETvDtKdQAAAABJRU5ErkJggg==';
$data = base64_decode($data);
$im = imagecreatefromstring($data);
As an alternative use any of the imagecreatefrom* functions if you know the image format in advance to load the URL directly, e.g.
imagecreatefrompng('http://example.com/image.png')