PHP page pagination

Yes, mixing PHP and HTML can get messy quite quickly.

If you do not want to switch to some templating engine, the easiest way to clean this up is to build the entire HTLM block in a PHP variable and then echo it:

$paginationHTML = '
<div class="row">
    <div class="twelve columns tac">';

if (isset($_GET["page_num"]) && $_GET["page_num"] >= 2) {
    $paginationHTML .= getPaginationLink($_GET["page_num"] - 1, "Back");
} else {
    $paginationHTML .= 'Back';
}

for ($i = 1; $i <= $totalPages; $i++) {
    if ($i == $_GET["page_num"]) {
        $paginationHTML .= $i;
    } else {
        $paginationHTML .= getPaginationLink($i, $i);
    }   
}

if ($_GET["page_num"] < $totalPages) {
    $paginationHTML .= getPaginationLink($_GET["page_num"] + 1, "Next");
} else {
    $paginationHTML .= 'Back';
}

$paginationHTML .= '
    </div>
</div>
';

echo $paginationHTML;

function getPaginationLink($number, $linkText) {
    return '<a href="?page=blog&page_num=' . htmlspecialchars($number, ENT_QUOTES, 'UTF-8') . '">' 
        . htmlspecialchars($linkText, ENT_QUOTES, 'UTF-8') . '</a>';
}

It is of course a matter of taste, but I find this a lot easier to read.

I also extracted some duplication to a function and removed the span tags which did not seem to have any purpose.

You could generalize the function even more, resulting in less code, but also possibly reduced readability:

$paginationHTML = '
<div class="row">
    <div class="twelve columns tac">';

$paginationHTML .= getPagination($_GET["page_num"] - 1, "Back", isset($_GET["page_num"]) && $_GET["page_num"] >= 2);

for ($i = 1; $i <= $totalPages; $i++) {
    $paginationHTML .= getPagination($i, $i, $i != $_GET["page_num"]);
}

$paginationHTML .= getPagination($_GET["page_num"] + 1, "Next", $_GET["page_num"] < $totalPages);

$paginationHTML .= '
    </div>
</div>
';

echo $paginationHTML;

// TODO comment
function getPagination($number, $linkText, $doLink) {
    if ($doLink) {
        return '<a href="?page=blog&page_num=' . htmlspecialchars($number, ENT_QUOTES, 'UTF-8') . '">' 
            . htmlspecialchars($linkText, ENT_QUOTES, 'UTF-8') . '</a>';
    } else {
        return htmlspecialchars($linkText, ENT_QUOTES, 'UTF-8');
    }
}

The XSS protection is not strictly necessary, but when writing a generic function which builds HTML, it's recommended, as the function may be reused in different contexts.

You should really consider using an orm like RedBean. In my own project, i got tired of writing hundreds of paginated queries. the methods were huge and complex and just a nightmare to manage. I replaced them all with two convenience functions:

// returns number of pages, current page, and results in an associative array.
public function paginateWithPageCount($sql,$params=[],$page=1, $per_page=20)
{
    //first we need to prepare our query to count the number of rows

    $sql2 = $sql; //copy sql at this point.
    // to calculate the total number of rows for the paginater, 
    // we need to delete the sql up to the `FROM` keyword
    $test = 'FROM';
    $n = strpos($sql2,$test);
    $keep = substr($sql2,$n+strlen($test));
    $sql2 = 'SELECT COUNT(id) AS num '.$keep;


    //we know that php is 0 based, but for presentation purposes pagination should be 1 based.
    //citing this knowledge, we always need to subtract `1` from $page;
    $page = (int) $page;
    if($page < 1){
        $page = 1;
    }
    $offset = abs($per_page * ($page - 1)); // page - 1 * per_page = offset

    //now we append our limit to the $sql var
    $sql.=' LIMIT '.$offset.','.$per_page;

    if(empty($params)){
        $data = \R::getAll($sql);
        $num = \R::getCell($sql2); //the getCell method returns the value of a single column of a single row query.
    }else{
        $data = \R::getAll($sql,$params);
        $num = \R::getCell($sql2,$params);
    }
    $num = $num > $per_page ? ceil($num/$per_page) : 1; //tada, now we have the total number of pages.

    return ['pages'=>$num,'current'=>$page,'results'=>$data];
}

//returns only results
public function paginate($sql,$params=[],$page=1, $per_page=20)
{
    //we know that php is 0 based, but for presentation purposes pagination should be 1 based.
    //citing this knowledge, we always need to subtract `1` from $page;
    $page = (int) $page;
    if($page < 1){
        $page = 1;
    }
    $offset = abs($per_page * ($page - 1)); // page - 1 * per_page = offset
    $sql.=' LIMIT '.$offset.','.$per_page;
    if(empty($params)){
        $data = \R::getAll($sql);
    }else{
        $data = \R::getAll($sql,$params);
    }
    return $data;
}