INFINITE FUNDS
The following code allows you to spend money without losing anything including upgrading staff
Press LEFT DOWN X CIRCLE (NOTE: this code must be entered 8 times inside the park for it to work you will here a distinctive sound to tell you that it has been correctly entered)

UNLOCK EVERYTHING
The following code allows you to unlock everything without needing to hire and train researchers for your park
Press UP DOWN UP DOWN LEFT UP DOWN UP DOWN RIGHT (NOTE: This code must be entered 8 times inside the park for it to work you will here a distinctive sound to tell you that it has been correctly entered) 

EXTRA GOLD TICKETS
The following code allows you to have 255 gold tickets and allows access to other parks straight away without playing the 1st park
Press UP DOWN LEFT RIGHT CIRCLE RIGHT LEFT DOWN UP CIRCLE (NOTE: this code must be entered 4 times inside the park for it to work you will here a distinctive sound to tell you that it has been correctly entered)

These codes will work and continue to work until you turn the game off if you wish to reuse the codes after switching the game off you must re enter the codes to which you wish to use 

arsenal1991 : Huh... Is this just for the original PS version? Or can these cheats be used in other versions such as the one here and the PSN version for PS3/PSP/Vita? If I can use those on those versions, I am definently going to try it out because it's my favorite Park building game of all time and I have almost all of the versions of this game.

This is for the PS1 version and should also work the same for Theme Park World (European version of the same game)

I just tested it out on my Vita and it worked, time to play the game all over again.

Using the infinite funds building the park up and then switching the game off then turning it back on is a great way to build and then see if you can make visitors happy