You should have searched first because it's not a rare question but anyway. Here is the code:
PHP Code:
#include <amxmodx>
#include <hamsandwich>
new g_amount
public plugin_init( )
{
RegisterHam( Ham_TakeDamage, "player", "fw_TakeDamage" );
g_amount = register_cvar("amx_damage_increase" , "1.5")
}
public fw_TakeDamage( victim, inflictor, attacker, Float:damage, damage_type )
{
if( is_user_alive( attacker ))
{
damage *= get_pcvar_float(g_amount)
SetHamParamFloat( 4, damage );
}
}
Example :
amx_cvar amx_damage_increase "1.5" - will multiply damage by 1.5 .