CollectiveAccess supports three widely used alphanumeric barcode formats:
It also supports numeric-only barcode formats:
Most barcode readers, even inexpensive ones, will read these codes. Barcodes can be output on labels for any field, but the two you are most likely to barcode with are [wiki:IDNumbers object ID numbers] and CollectiveAccess internal ID numbers. The latter are guaranteed-unique numbers CA uses to track objects internally.
Unless you are sure the values you are outputting are numeric-only you should use one of the alphanumeric formats. If you are not sure which format to use Code 128 is recommended. It is the most space-efficient format and thus allows the most information to be encoded. 3of9 is a widely used format that is suggested if your barcode reader doesn't support Code 128 (unlikely) or you are already using 3of9 format codes.
Your PHP installation must support the GD graphics library for barcoding to work. Fortunately, GD is present on most PHP installations, so this should rarely be a problem.
See Printable_Labels sphinx