Qsprintf

qsprintf <quote1> <quote2> <gamevar1> [gamevar2] [gamevar3] ... [gamevar32]

Copies the text of <quote2> to <quote1>, replacing all variable placeholders %ld with their gamevar values in relative order.

You can use a single quote for both placeholders. Also, you can use a single quote to display many different texts at the same time, since using qsprintf in a quote doesn't alter the text displayed by a gametext command used before with that quote.

gamevar TEMP 0 1
definequote 254 PLACEHOLDER

onevent EVENT_DISPLAYREST
getplayer[THISACTOR].actors_killed TEMP // Get the number of kills the player has

redefinequote 254 ALIENS KILLED: %ld    // Redefine a placeholder quote
qsprintf 254 254 TEMP                   // Place our variable into the %ld string
gametext STARTALPHANUM 10 10 254 0 0 0 ZERO ZERO xdim ydim // And print to the upper left!
endevent