Call to undefined function sqlsrv_connect() when connecting Microsoft SQL management studio 2014 with php

Question

PHP Fatal error: Uncaught Error: Call to undefined function sqlsrv_connect().

$serverName = "serverName";

$connectionInfo = array("Database" => "databaseName");
$conn = sqlsrv_connect($serverName, $connectionInfo);

if ($conn) {
echo "Connection established.<br />";
} else {
echo "Connection could not be established.<br />";
die(print_r(sqlsrv_errors(), true));
}