PHP - What Is A Php Code To Automatically Bold Certain Words Within My Content?

I've been working on this html/php page for class and I just don't understand why my code is acting like this.
I have this form that gets the user input and then it validates the code with appropriate error messages but even when I test it and type in something for each textbox it automatically makes the textbox go blank again and displays the error messages and i dont know why.

This is my code

<?php
//error_reporting(E_ALL);
//ini_set('display_errors', 1);

$title = "Listing Search";
$file = "listing-search.php";
$description = "Listing Search page for real estate website (WEBD3201)";
$date = "2019-10-01";
$banner = "Listing Search page";
require "header.php";

?>

<?php
$headline = "";
$minPrice = "";
$maxPrice = "";
$city = "";

$error = "";
$output = "";
$error = "";
$output = "";

if(isset($_GET["city"]))
{
        $city = $_GET["city"];
        setcookie('city',$city,COOKIE_LIFESPAN);
        $_SESSION['city'] = $city;
}

else if (isset($_COOKIE['city']))
{
        $city = $_COOKIE['city'];
        $_SESSION['city'] = $city;
}

if(!isset($city))
{
        $_SESSION['RedirectError'] = "Please select a city<br/>";
        header("Location:listing-select-city.php");
}


if(isPost())
{
      $minPrice = trim($_POST["minPrice"]);
        $output .=$_POST["minPrice"];
        $maxPrice = trim($_POST["maxPrice"]);
        $output .=$_POST["maxPrice"];
        $headline = trim($_POST["headline"]);
        $output .=$_POST["headline"];
        $city = trim($_POST["city"]);
        $output .=$_POST["city"];

        $error = "";
        $output = "";

        if($headline == "")
        {
            $error .= "<br/>Headline was not specified";
        }
        if($city == "")
        {
            $error .= "<br/>City was not specified";
        }

        if($minPrice == "")
        {
         $error .= "<br/>Minimum price was not specified";
      }
        elseif (preg_match(PRICE_FILTER, $minPrice) == '0')
        {
        $error .= "<br/>Minimum price needs to be a number";
        $minPrice = "";
        }
        if($maxPrice == "")
        {
            $error .= "<br/>Maximum price was not specified";
        }
        elseif (preg_match(PRICE_FILTER, $maxPrice) == '0')
        {
            $error .= "<br/>Maximum price needs to be a number";
            $minPrice = "";
      }
             if($maxPrice < $minPrice)
             {
                 $error .= "<br/>Minimum price must be smaller than maximum price";
             }

}
 ?>

 <?php echo $error; ?>
 <?php echo $output;?>

<h2>Search for a home.</h2>

<form method="post" action="<?php echo $_SERVER['PHP_SELF']; ?>">
    <table style="margin-left: auto; margin-right: auto;">
    <tr>
        <td>
            Listing Name
        </td>
        <td>
            <input type="text" name="newlist" value="<?php echo $headline ?>" />
        </td>
        <td>
            City
        </td>
        <td>
            <input type="text" name="city" value="<?php echo $city ?>" />
        </td>
    </tr>
    <tr>
        <td>
            Min Price
        </td>
        <td>
            <input type="text" name="minPrice" value="<?php echo $minPrice ?>"  />
        </td>
            <td>
                Max Price
            </td>
            <td>
                <input type="text" name="maxPrice" value="<?php echo $maxPrice ?>"  />
            </td>
        </tr>
    </table>
    <table style="margin-left: auto; margin-right: auto;">
    <tr>
        <td>
            <input type="submit" value = "Search" />
        </td>

        <td>
            <input type="reset" value = "Clear" />
        </td>
    </tr>
    <tr>
        <td>
            <?php echo build_checkbox("internet", "DSL"); ?>
        </td>
        <td>
            <?php echo build_checkbox("heating", ""); ?>
        </td>
    </tr>
    </table>
</form>

<?php
require "footer.php";
?>