<?php

$store = array(1=>"101Phones - Product Catalog TXT",2=>"1-800-FLORALS - Product Catalog 1");

foreach( $store as $key => $value){
    $store[$key] = $value.".txt.gz";
}

print_r ($store);

?>

Array
(
    [1] => 101Phones - Product Catalog TXT.txt.gz
    [2] => 1-800-FLORALS - Product Catalog 1.txt.gz
)