wordpress中的错误插入到自定义表值为空或null

i want to get title of all post from wordpress post and insert into a custom table but it always insert null value why

<?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'));
}
?>

In your function call you're not passing the post title as an argument.

insert_p_now(the_permalink());

It means that in your function

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

The value of $a is equal to the_permalink() which isn't your post_title of course. If all you want to do is store the post title in a custom array, perhaps this is what you're looking for:

<?php

global $wpdb;

$args = array( 'numberposts' => -1 );
$allposts = get_posts( $args );

foreach ( $allposts as $curpost ) : 
   $wpdb->insert('wp_posttable', array('post_title' => $curpost->post_title), array('%s')); 
endforeach;

?>

You can of course use a separate function if you'll be reusing the "insert into custom table" functionality.

Hope that helps!