Stack of codes
PHP: Hypertext Preprocessor (or simply PHP) is a server-side scripting language designed for web development but also used as a general-purpose programming language. It was originally created by Rasmus Lerdorf in 1994.
The PHP interpreter only executes PHP code within its delimiters. Anything outside its delimiters is not processed by PHP, although non-PHP text is still subject to control structures described in PHP code. 
The most common delimiters are to close PHP sections. The shortened form <? also exists. This short delimiter makes script files less portable, since support for them can be disabled in the local PHP configuration and it is therefore discouraged.However, there is no recommendation against the use of the echo short tag <?=Prior to PHP 5.4.0, this short syntax for echo() only works with the short_open_tag configuration setting enabled, while for PHP 5.4.0 and later it is always available. The purpose of all these delimiters is to separate PHP code from non-PHP content, such as JavaScript code or HTML markup
mysqli_error() needs you to pass the connection to the database as a parameter. Documentation here has some helpful examples:
Try altering your problem line like so and you should be in good shape:
$query = mysqli_query($myConnection, $sqlCommand) or die (mysqli_error($myConnection));
<button onclick="myFunction()">Try it</button>
<script>
function myFunction() {
alert("I am an alert box!");
}
</script>
