MySQL+PHP+WP: I need to create a Contacts record when the user click Submit

Thanks. I read your three links, but am unsure about the second link and the purpose of that long function which is contained in the PHP file...
File: wp-includes/wp-db.php

Thanks. Here is my test, but I find it difficult, with no way to output status info. You see, page #37 loads the plugin, as do other pages, but I can not pause the load, and any statement I "echo", vanishes. Suggestion on forcing it to pause?

Also, this code does not insert a record, which means I am stuck, not knowing why it failed.

<?php
/**
 * Hello World! for WordPress
 *
 * 
 */


add_action( 'wp_enqueue_scripts', 'ajax_signup_enqueue_scripts' );
function ajax_signup_enqueue_scripts() {   
    global $wp_query;
    $page_id = $wp_query->post->ID;
    $page_name = $wp_query->post->post_title;
    
    $page_title = trim($page_name);
    
    if(isset($page_title) === false || $page_title === '') {
        // The title is empty
        // Throw Fatal Error()   
    }
    if(is_page()){ //Check if we are viewing sign up page
        if($page_id === 37){  
            global $wpdb;
            
            $fname = 'King '. $wpdb->prefix;
            $lname = 'Kong!';

            $table_name = "ps_contacts";

            if ( $wpdb->insert(
                $table_name,
                array(
                    'fname' => $fname,
                    'lname' => $lname,
                ) != false ) 
               // echo here fails, even when enclosed in {}
               //echo "SUCCESS! SUCCESS! SUCCESS! SUCCESS! SUCCESS! SUCCESS! SUCCESS! SUCCESS! SUCCESS!";
            );          
        } 
    }
}
?>

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Ok, the logging helps a lot. Thanks.

I THOUGHT it was not getting into the function, since the only output to the debug.log was "Step 0".

error_log("Step 0", 0);

add_action( 'save_post', 'ajax_signup_insert_contact' );
function ajax_signup_insert_contact() {  
    error_log("Step 1", 0);
    global $wp_query;
    $page_id = $wp_query->post->ID;
    $page_name = $wp_query->post->post_title;    
   

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Then, I check the database and found these records:

So, it seems the Insert statement works, but there is something changed about the debug.log. Any thought on that before I close this question?

Thanks

I am not sure but it looks like you are making good headway

Now it stopped working, where there is no logging AND the DB does not get updated. See any issue with the code before I close out this question?

error_log("Step 0", 0);

add_action( 'save_post', 'ajax_signup_insert_contact' );
function ajax_signup_insert_contact() {   
    error_log( "Step 1", 0);
    global $wp_query;
    $page_id = $wp_query->post->ID;
    $page_name = $wp_query->post->post_title;    
    
    error_log( "Step 2", 0);
    if(is_page()){ //Check if we are viewing sign up page
        error_log( "Step 3", 0);
        if($page_id === 37){  
            error_log("Step 4", 0);
            global $wpdb;
            
            $fname = 'C';
            $lname = 'D';

            $table_name = "ps_contacts";

            if ( $wpdb->insert(
                $table_name,
                array(
                    'fname' => $fname,
                    'lname' => $lname,
                )                
            ) == false)
            {
                error_log( "FAILED", 0);
            }
        }
    }

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In the code you posted, you are not closing line 4,

function ajax_signup_insert_contact() {  

I think that was a copy problem. Here is the final version:

add_action( 'save_post', 'ajax_signup_insert_contact' );
function ajax_signup_insert_contact() {   
    error_log( "Step 1", 0);
    global $wp_query;
    $page_id = $wp_query->post->ID;
    $page_name = $wp_query->post->post_title;    
    
    /*if(isset($page_title) === false || $page_title === '') {
        // The title is empty
        // Throw Fatal Error()   
    }*/
    error_log( "Step 2", 0);
    if(is_page()){ //Check if we are viewing sign up page
        error_log( "Step 3", 0);
        if($page_id === 37){  
            error_log("Step 4", 0);
            global $wpdb;
            
            $fname = 'C';
            $lname = 'D';

            $table_name = "ps_contacts";

            if ( $wpdb->insert(
                $table_name,
                array(
                    'fname' => $fname,
                    'lname' => $lname,
                )                
            ) == false)
            {
                error_log( "FAILED", 0);
            }
        }
    }
}

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I am in the midst of debugging, since I started to see errors show up in the debug.log. So, I may post new questions, shortly...Thanks.

I am still stuck on whether the action is firing or the logging does not work inside the action. Here is my question...

https://www.experts-exchange.com/questions/29218471/WordPress-PHP-has-me-stuck-with-no-logging.html#questionAdd