<?php

function minify($html) {
    $lines = explode(PHP_EOL, $html);
    array_walk($lines,
function(&$line) {
        $line = trim($line);
    });
    
    $lines = array_filter($lines, function($line) {
        return $line !== '';
    });

    return implode(PHP_EOL, $lines);
}

$html = '<div class="container">

    <div class="row">

        <div class="col-sm-12">
            //content here
        </div>

    </div>

</div>';

$minify = minify($html);
echo ($minify);