establish a connection for your sql database and select the database initially

sql_conn.php

<?php
  //SQL CONNECTION
	$url = '';
	$db_user = '';
	$db_pass = '';
	$db = '';

	$conn = mysqli_connect($url, $db_user, $db_pass, $db);

	if(!$conn){
		echo "<center>Connection not established, please check your code.<br>";

		echo "<br>Debugging errno: " . mysqli_connect_errno() . PHP_EOL;
    	echo "<br>Debugging error: " . mysqli_connect_error() . PHP_EOL;
	 }else{
	// 	echo "Connected to " . $url . " " . $db_user . " " . $db . " " ;
	 	mysqli_query($conn, "USE $db;");
	}
  ?>