PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

490 Chapter 22 Generating Images
This function sends raw HTTP header strings. Another typical application of this
function is to do HTTP redirects.They tell the browser to load a different page instead
of the one requested.They are typically used when a page has been moved. For example,
Header (‘Location: http://www.domain.com/new_home_page.html ‘);
An important point to note when using the Header() function is that it cannot be executed
if content has already been sent for the page. PHP will send an HTTP header
automatically for you as soon as you output anything to the browser. Hence, if you have
any echo statements, or even any whitespace before your opening PHP tag, the headers
will be sent, and you will get a warning message from PHP when you try to call
Header(). However, you can send multiple HTTP headers with multiple calls to the
Header() function in the same script, although they must all appear before any output is
sent to the browser.
After you have sent the header data, you output the image data with a call to
imagepng ($im);
This call sends the output to the browser in PNG format. If you wanted it sent in a different
format, you could call imagejpeg()—if JPEG support is enabled.You would also
need to send the corresponding header first, as shown here:
Header (‘Content-type: image/jpeg’);
The second option you can use, as an alternative to all the previous ones, is to write the
image to a file instead of to the browser.You can do this by adding the optional second
parameter to imagepng() (or a similar function for the other supported formats):
imagepng($im, $filename);
Remember that all the usual rules about writing to a file from PHP apply (for example,
having permissions set up correctly).
Cleaning Up
When you’re done with an image, you should return the resources you have been
using to the server by destroying the image identifier.You can do this with a call to
imagedestroy():
imagedestroy($im);
Using Automatically Generated Images in Other
Pages
Because a header can be sent only once, and this is the only way to tell the browser that
you are sending image data, it is slightly tricky to embed any images you create on the
fly in a regular page.Three ways you can do it are as follows:
n
You can have an entire page consist of the image output, as we did in the previous
example.