Image Processing and Creation in PHP - How to Create 300DPI Images

When creating images using the GD library in PHP ie) imagecreatetruecolor()what is the DPI of the resulting image? I could not find anyone by specifying where to configure dpi or the default.

I need to create a tiff or jpeg 300DPI that will be created and then saved to the file system from a program.

If this is not possible with the GD library, is there another option for this situation?

thanks for the help

Edit: Yes, this creates an image. I want the 300dpi file saved from the script to not have to open every file in Photoshop to indicate dpi.