select count(*) from table of mysql in php

Please start using PDO.

mysql_* is deprecated as of PHP 5.5.0 and will be removed entirely in 7. Let's make it easier to upgrade and start using it now.

$dbh = new \PDO($dsn, $user, $password);
$sth = $dbh->prepare('SELECT count(*) as total from Students');
$sth->execute();
print_r($sth->fetchColumn());

If you only need the value:

$result = mysql_query("SELECT count(*) from Students;");
echo mysql_result($result, 0);

$result = mysql_query("SELECT COUNT(*) AS `count` FROM `Students`");
$row = mysql_fetch_assoc($result);
$count = $row['count'];

Try this code.

You need to alias the aggregate using the as keyword in order to call it from mysql_fetch_assoc

$result=mysql_query("SELECT count(*) as total from Students");
$data=mysql_fetch_assoc($result);
echo $data['total'];