PHP - Need Help With Natural Log

I need help with these 3 formulas. Since PHP doesn't accept the LN function. I appreciate the help.

              1.162*ln('PREBUN'/'POSTBUN') 
 

              -ln(('POSTBUN'/'PREBUN)-(0.008*('TIME'/60)))+((4-(3.5*'POSTBUN'/'PREBUN'))*'UFVOL'/'POSTWEIGHT')

              ln('PREBUN'/'POSTBUN')