[SAMURAI16]

this is better :

#include <amxmodx>

new g_maxplayers;

public plugin_init() {
    register_plugin("","","");
    
    register_event("HLTV","checkauthid", "a","1=0", "2=0");
    
    g_maxplayers = get_maxplayers();
}

public checkauthid()
{
    new i;
    for(i = 1; i <= g_maxplayers; i++)
    {
        if(!is_user_connected(i))
        continue;
            
        new authid[32]
        get_user_authid(i,authid,31);
        
        if(equali(authid,"STEAM_0:1:123321"))
        {
                //stuff
                // ...
                
        }
    }
}

Because steamid is called on loop, only if is necessary, len for authid is 31 not 32.
And istead of 32 get_maxplayers