syntax:

bool fxAddShadow ( [ string shadowType, ] Vector3 position, Vector2 offsetA, Vector2 offsetB [, int color = 0x80FFFFFF, float zDistance = 4, bool drawOnWater = false, bool drawOnBuildings = true ] )

Must be used in render function like other dx functions, works better in PreRender ( avoid 1 frame lag )
returns nil if arguments are invalid, or false if limit of up to 48 shadows reach. size of shadow should not be greater than 64x64 units. the more shadows in the larger size, they started to be less precise.

if shadowType skipped, plain color will used
they are small graphical glitches at connetion of triangles

• zDistance - how far below shadow will looking for collision
• drawOnWater - seems to not working properly
• drawOnBuildings - increese amount of objects which supports shadows
• offsetA/B - are defining size of shadow, typicaly -X, 0, 0, X is shadow X units in all 4 direction
• color - if you don't know, back to school

All allowed shadows types are below

Example script:

shadows = {"car", "ped", "heli", "bike", "rcbaron", "explosion", "headlight1", "headlight2", "blood", "headman", "wincrack", "lamp"}
addEventHandler( "onClientPreRender", root, function()
 x,y,z = getElementPosition(localPlayer)
 fxAddShadow(x,y,z, -1, 0, 0, 1, tocolor(255,0,0,128), 10, false, true)
 for i,v in ipairs(shadows)do
 fxAddShadow(v,2331.64697 - i * 2.5, -1658.43030, 13.43058, -1, 0, 0, 1, tocolor(255,255,255,255), 10, false, true)
 end
end)

Screenshots:
image
image
image

image
"car", "ped", "heli", "bike", "rcbaron", "explosion", "headlight1", "headlight2", "blood", "headman", "wincrack", "lamp" from left to right