Why row always empty wordress custom table

CMS & WordPress
1

my website is : http://designspicy.com/ i am try to retrive all post from wordpress and insert into a custom table but it always insert null value why

my code is here

<?php
global $post;
$args = array( 'numberposts' => -1 );
$myposts = get_posts( $args );
foreach ( $myposts as $post ) : 
setup_postdata( $post ); 
insert_p_now(the_permalink());
endforeach;
wp_reset_postdata(); 

function insert_p_now($a)
{
    global $wpdb;
    $wpdb->insert('wp_posttable', array('post_title' => $a), array('%s'));
}
?>