Convertir Código visual a PHP

La funcion original esta en Fox, La misma fue convertida a Visual , y Ahora se necesita converir la misma a PHP Public Function digverif(ByVal cadena As String) As String ' muni zapala codigo original que mando esta hecho en FOX retorna 2 digitos ' LPARAMETERS [login to view URL] ' [login to view URL] = 0 ' secuencia = 3 ' [login to view URL] = SUBSTR([login to view URL], 1, 1) ' [login to view URL] = Val([login to view URL]) * 1 ' [login to view URL] = [login to view URL] + ([login to view URL]) ' For [login to view URL] = 2 To Len([login to view URL]) Step 1 ' [login to view URL] = SUBSTR([login to view URL], [login to view URL], 1) ' [login to view URL] = Val([login to view URL]) * secuencia ' [login to view URL] = [login to view URL] + ([login to view URL]) ' secuencia = secuencia + 2 ' if secuencia=11 ' secuencia = 3 ' End If ' Next [login to view URL] ' [login to view URL] = MOD(int([login to view URL]), 10) ' [login to view URL] = mod(int((m.suma+([login to view URL]*secuencia))/2),10) ' [login to view URL] = [login to view URL] + alltrim(Str([login to view URL])) + alltrim(Str([login to view URL])) ' [login to view URL] = [login to view URL] ' RETURN [login to view URL] ' CODIGO CONVERTIDO A VISUAL BASIC suma = 0 secuencia = 3 letra = Mid(cadena, 1, 1) valorreal = Val(letra) * 1 suma = suma + (valorreal) For co = 2 To Len(cadena) Step 1 letra = Mid(cadena, co, 1) valorreal = Val(letra) * secuencia suma = suma + (valorreal) secuencia = secuencia + 2 If secuencia = 11 Then secuencia = 3 End If Next co checksum = Int(suma / 2) Mod 10 checksum2 = Int((suma + (checksum * secuencia)) / 2) Mod 10 digverif = Trim(Str(checksum)) + Trim(Str(checksum2)) End Function