<form action="index.php" method="POST">
    <input type="text" placeholder="Įveskite a" name="sideA">
    <input type="text" placeholder="Įveskite b" name="sideB">
    <button type="submit" name="submitForm">Įvesti</button>
</form>

<?php
    if(isset($_POST['submitForm'])) {
        $sideA = $_POST['sideA'];
        $sideB = $_POST['sideB'];
        $sideC = sqrt(($sideA * $sideA) + ($sideB * $sideB));
        $trianglePerimeter = $sideA + $sideB + $sideC;
        $triangleSpace = ($sideA * $sideB) / 2;

        echo "--------------------------<br>";
        echo "Kraštinė a: {$sideA}<br>";
        echo "Kraštinė b: {$sideB}<br>";
        echo "Kraštinė c: {$sideC}<br>";
        echo "--------------------------<br>";
        echo "Plotas: {$triangleSpace}<br>";
        echo "Perimetras: {$trianglePerimeter}<br>";
        echo "--------------------------<br>";
    }
?>