How do I convert a WKT PRJ File or String to a Proj4 String?

GDAL provides a tool called "gdalsrsinfo" where information about a given SRS can be displayed in a number of formats. If you have an WKT PRJ (OGC/Human Readable OGC/ESRI) file or string then you can use this tool to convert it to a proj4 string.

Example command prompt:

gdalsrsinfo D:\EPSG\28356_wkt.prj -o proj4

Contents of PRJ file is a Human Readable OGC WKT: https://spatialreference.org/ref/epsg/28356/prettywkt/

Results displayed in command prompt:

+proj=utm +zone=56 +south +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

More examples and help can be found here https://gdal.org/programs/gdalsrsinfo.html

Note: if the -v (Validate SRS) argument is used it may create a "Failed" even though it has output.