proper construction of a PHP html string inside of a foreach loop, not looping

Question

Using the code that I have below, I am getting the $cid to loop but the $html_string is not looping. It seems to only run once. Why?

php

  $query = $wpdb->get_results("SELECT firstname, lastname, id FROM table_name");

add_action("wp_ajax_myphp", "myphp");   
add_action("wp_ajax_myphp", "myphp");
function myphp() {          
global $query;

$html_string = '';
foreach($a as $value){
 
$html_string = ['<div class=\"myclass\">'.$value-> firstname.'</div><div class=\"myclass\">'.$value-> lastname.'</div>'];
$cid[] = $value->id;
    }; 
    
    
echo json_encode(array('html' => $html_string, 'cid' => $cid));
    die;
}

Jquery

jQuery.ajax({
     data: {action: 'myphp'},
     type: 'post',
     url: my_ajax.ajax_url,
     dataType: 'JSON',
     success: function(data) {  
         console.log(data);
     }
});

sample output from the above code:

cid: Array(3) [ "44", "45", "46" ]
​html: (1) […]
​​
0: "<div class=\\\"myclass\\\">Bob</div><div class=\\\"myclass\\\">Smith</div>"
​​
length: 1

It iterates over the cid three times just as it should but only once

Answer 1

$html_string isn't an array, so it contains the value of the last iteration.

Try

$html_string[] = '<div class=\"myclass\">'.$value-> firstname.'</div><div class=\"myclass\">'.$value-> lastname.'</div>';

and remove

$html_string = '';