$say=0;
    
    for($i=0; $i < 100; $i++){
        
        if(0== $say){
            
            echo '<div class = "class1">Class1</div>';
            echo '<div class = "class1">Class1</div>';
            echo '<div class = "class1">Class1</div>';
            echo '<div class = "class1">Class1</div>';
            $say++;
        }else{
            echo '<div class = "class2">Class2</div>';
            echo '<div class = "class2">Class2</div>';
            echo '<div class = "class2">Class2</div>';
            echo '<div class = "class2">Class2</div>';
            $say=0;
            
        }

    }