How would I split the results into two columns [closed]

Something like this should work though I havent testested it:

PHP:

<?php

$inputs="<ul class="checkbox-columns">";

foreach($subjects as $subject){
    $inputs .= '<li><label><input type="checkbox" name="subjects[]" value="' . $subject['subject'] . '"/>' . $subject['subject'] . '</label></li>';
}

$inputs .= '</ul>';

echo $inputs;

CSS:

.checkbox-columns {
    width: 200px;
}
.checkbox-columns li {
    display: block;
    float: left;
    width: 50%;
}

This creates a variable called $inputs containing a string with an opening <ul> tag with a CSS class of .checkbox-columns. It then loops over an array of $subjects and appends a string of list items (<li>) containing the inputs (with it’s value attribute and label text set to the value of $subject['subject']) to the $inputs variable. Finally, when the loop is finished it appends a closing </ul> tag and echos out the string of HTML that was built.

Then you can style the output with some CSS as needed.

A few notes on the code you posted:

• Instead of only using double quotes (e.g "), it’s a bit easier to read if you alternate between single and double quotes (e.g " and '). There is also no need to escape the HTML attribute’s quotes this way.
• In the foreach parameters, AS should be lowercase.
• I wrapped the whole <input /> tag in a <label> tag. This lets you add the label text inside HTML instead of just dumping it into a random text node.
• Depending on where the $subjects array comes from you should strongly consider Sanitizing the output.