PHP - Multiple Queries With Mysqli

<?php 

// Get required login info
include "/path/to/login/info/file.php"; // Get required login info - changed for this post.

$db = new mysqli('localhost', $username, $password, $database); // Connect to DB using required login info
if($db->connect_errno > 0){
    die('Unable to connect to database [' . $db->connect_error . ']');
}

unset($username);// put these variables back to null
unset($password);// put these variables back to null
unset($database);// put these variables back to null


//query db
$sql = <<<SQL
    SELECT *
    FROM `ft_form_7`
	WHERE DATE(class_start_date) >= CURDATE()
	OR DATE(class_end_date) >= CURDATE()
	ORDER BY class_start_date ASC
SQL;

if(!$result = $db->query($sql)){ // if there is an error in running the query, show error message.
    die('There was an error running the query [' . $db->error . ']');
}

while($row = $result->fetch_assoc()){
	// Get start date information
	$start_date = $row['class_start_date']; // Get event_start_date for conversion and call it $start_date
	$start_date_formatted = date("l M d, Y", strtotime($start_date)); // Convert start_date 
	$end_date = $row['class_end_date']; // Get event_end_date for conversion and call it $start_date
	$end_date_formatted = date("M d, Y", strtotime($end_date)); // Convert start_date
	
	// Get time information.
	$start_time = $row['class_start_time']; // Get event_start_time for conversion and call it $start_time
	$start_time_formatted = date("h:i A", strtotime($start_time)); // Convert start_time 
	$end_time = $row['class_end_time']; // Get event_end_time for conversion and call it $end_time
	$end_time_formatted = date("h:i A", strtotime($end_time)); // Convert end_time 
	
	// echo information...
	    echo "<h2>" , $row['class_name'],"</h2>" ; // echo event name
	    echo "<p><strong>",$start_date_formatted; // echo the start date
		if (empty($start_time)) { echo ''; } else { echo " (", $start_time; } // echo start time
		if (empty($end_date)) { echo ''; } else { echo " -","<br />", $end_date_formatted; } // echo end date
		if (empty($end_time)) { echo ')'; } else { echo " - ", $end_time, ")"; } // echo end time
	// if there is no start time, echo nothing. (otherwise it seems to echo 4pm). If it does contain a time, echo the time.
	echo "</strong><br />";

	$chef = $row['Instructor'];
	global $chef;
	
	if ($chef != NULL) {
		require ('instructors.php'); }
	echo $row['class_description'], "<br />";
	echo "<strong>" , $row['type'], "&nbsp;-&nbsp;Cost: &#36;",$row['cost'] , " - #" , $row['course_number'] , "</strong><br />" , "</p>"; // echo class type and cost
}

$db->close();
$result->free();

?>

<?php

include "/path/to/login/info/file.php"; // Get required login info - changed for this post.

$db_instructors = new mysqli('localhost', $username, $password, $database); // Connect to DB using required login info
if($db_instructors->connect_errno > 0){
    die('Unable to connect to database [' . $db_instructors->connect_error . ']');
}

unset($username);// put these variables back to null
unset($password);// put these variables back to null
unset($database);// put these variables back to null

//query db
$sql_instructors = <<<SQL
    SELECT *
    FROM ft_form_8
    WHERE chef_name = '$chef'
SQL;

if(!$result_instructors = $db_instructors->query($sql_instructors)) { // if there is an error in running the query, show error message.
    die('There was an error running the query [' . $db_instructors->error . ']');
}

	while($row_instructors = $result_instructors->fetch_assoc()) {
	$chef_full_name = $row_instructors['chef_name'];
	$chef_id = $row_instructors['submission_id'];
	$full_bio = $row_instructors['full_bio'];
	echo "<a href=\"#\" class=\"clickme\">" , $chef_full_name , "</a>";
	echo "<div class=\"box\">";
	echo $full_bio , "</div>";
}


$db_instructors->close();
$result_instructors->free();

?>

$error=array();
$sql = 'INSERT INTO staff(username, email, phone)
VALUES (?, ?, ?)';
$stmt = $conn->stmt_init();
$stmt = $conn->prepare($sql);
// bind parameters and insert the details into the database
$stmt->bind_param('sss', $username, $email, $phone);
$stmt->execute();
if ($stmt->errno == 1062) {
$errors[] = "One of the fields is already in use.";
}

Good day everyone. I go by the nickname Sbosh and I am a newbie in PHP.

I am currently learning php using video tutorials. I need help with regards to php queries.

I have the following code:

"SELECT 'food', 'calories' FROM 'diet' ORDER BY 'id'"

which is supposed to display content from a table inside phpmyadmin which i created manually using phpmyadmin. But It gives an error saying i must check the MariaDB version on how to right this query, something like that.

Please assist on the proper way of writing this code. Again I am a newbie in PHP and just starting to learn, so your help will be highly appreciated.

<?php
function childtheme_cat_limited_blog( $query ) {
    if ( $query->is_home() && $query->is_main_query() ) {
        $query->set( 'events_categories', 'mycategory1' );
    }
}
add_action( 'pre_get_posts', 'childtheme_cat_limited_blog' );
?>

<?php
function childtheme_cat_limited_blog( $query ) {
    if ( $query->is_home() && $query->is_main_query() ) {
        $query->set( 'category_name', 'mycategory2' );
    }
}
add_action( 'pre_get_posts', 'childtheme_cat_limited_blog' );
?>