if ([FM] == 'S') $b1 = -0.313; else
if ([FM] == 'R') $b1 = -0.117; else
$b1 = -0.242;
$b2 = 0.527;
$b3 = 0.000;
$b5 = -0.778;
$bV = -0.371;
$VA = 1396;
$h = 5.57;
return exp($b1+$b2*([Mw]-6)+$b3*pow([Mw]-6, 2)+$b5*log([de:km])+$bV*log([Vs30:m/s]/$VA));

Validation:
M = 5.0, d = 10 km, FM = 'S', Vs30 = 800 : PGA = 0.088490926097055 g
M = 6.0, d = 10 km, FM = 'S', Vs30 = 800 : PGA = 0.14988974894016 g
M = 5.0, d = 100 km, FM = 'S', Vs30 = 800 : PGA = 0.014753624987138 g