imageloadfont() loads a user-defined bitmap
font and returns an identifier for the font (that is always
greater than 5, so it will not conflict with the built-in fonts).
It returns FALSE in case of error.
The font file format is currently binary and architecture
dependent. This means you should generate the font files on the
same type of CPU as the machine you are running PHP on.
Table 1. Font file format
C data type
number of characters in the font
value of first character in the font (often 32 for space)
pixel width of each character
pixel height of each character
array with character data, one byte per pixel in each
character, for a total of (nchars*width*height) bytes.