How to make search and replace in content through php

From the documentation of WP All Import, the pmxi_saved_post action takes one parameter, $id, which is “the ID of the post/page/Custom Post Type that was just created.”

So, your post_saved() callback should look like this:

add_action('pmxi_saved_post', 'wpse246838_post_saved', 10, 1);
function wpse246838_post_saved( $id ) {
    $my_post = array(
            "ID"           => $id,
            "post_content" => "1",
    );
    wp_update_post( $my_post );
}