"no database selected" error even query is fine - php

Question

I am new to mysql and php. I am working on database programming with php with mysql but getting "no data base selected" error continuously. I found this error quite famous on internet. I tried every answer which were given to others having the same problem but nothing worked.
Here is my code:
if(!#mysql_connect('localhost','root','') || !#mysql_select_db ('a_database') ){
die ('Connection Error !');
}
$query = "SELECT `food`,`calories` FROM `food` ORDER BY `id`";
if($query_run=mysql_query($query)){
while($query_row = mysql_fetch_assoc($query_run))
{
$food = $query_row['food'];
$calories = $query_row['calories'];
echo $food.' has '.$calories.' Calories'.'<br>';
}
} else {
echo mysql_error();
}
This is the code that gives error. After some search on net. I made some bit of changes, but the result was same.
Changes that I made to first 3 to 4 lines:
$link = mysql_connect('localhost','root','');
if(!$link || !mysql_select_db ('a_database', $link) ){
die ('Connection Error !');
}
Please tell me what should I do to get rid of this problem, Thank you.

Answer 1

Try with this
<?php
// Create connection
$con=mysqli_connect("localhost","root","","a_database");
// Check connection
if (mysqli_connect_errno($con))
{
echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
?>

Answer 2

Try this
$mysqli = new mysqli("localhost", "user", "password", "database");
if ($mysqli->connect_errno) {
echo "Failed to connect to MySQL: (" . $mysqli->connect_errno . ") " . $mysqli->connect_error;
}
echo $mysqli->host_info . "\n"; // if everything is successful
And yes don't use mysql_* as it's deprecated, use mysqli_ or PDO.