PC Cheats:

All weapons:
1 - Select 9mm pistols
2 - One step forward
3 - One step back
4 - Crouch
5 - Stand up
6 - Turn around 3 times in either direction
7 - Back flip

Skip level:
1 - Select 9mm pistols
2 - One step forward
3 - One step back
4 - Crouch
5 - Stand up
6 - Turn around 3 times in either direction
7 - Jump forward

PS1 Cheats:

All weapons:
1 - L2, R2(2x), L2(4x)
2 - R2, L2, R2(2x), L2
3 - R2(2x), L2(2x), R2
4 - L2(2x), R2

Skip level:
1 - L2, R2, L2(2x), R2
2 - L2, R2, L2, R2, L2(4x)
3 - R2, L2, R2(4x), L2

All secrets:
1 - L2(5x), R2, L2(3x), R2
2 - L2, R2, L2(2x),R2, L2(2x)
3 - R2, L2(2x)

Fill up your live:
1 - R2(2x), L2, R2, L2(6x), R2
2 - L2(3x), R2,L2(5x)

Quad's Key (Croft Manor):
1 - R2, L2(3x), R2, L2(6x),R2
2 - L2(5x), R2, L2(2x)