<?php

$a = array( 'truck', 'vehicle', 'seddan', 'coupe' ); 

$str = 'Honda is a truck. Toyota is a vehicle. Nissan is a sedan. Scion is a coupe.'; 

echo str_replace($a,'car',$str); 

?>

Honda is a car. Toyota is a car. Nissan is a sedan. Scion is a car.