Something

<?php
$a = -3.6;
$x1 = 1;
$x2 = 2;
$x3 = 3;
$x4 = 4;

$y1 = -1.3;
$y2 = -9.8;
$y3 = -25.5;
$y4 = -48.4;

$sqx1 = $x1*$x1;
$sqx2 = $x2*$x2;
$sqx3 = $x3*$x3;
$sqx4 = $x4*$x4;

$ax1 = $a*$sqx1;
$ax2 = $a*$sqx2;
$ax3 = $a*$sqx3;
$ax4 = $a*$sqx4;

$dya1 = $y1-$ax1;
$dya2 = $y2-$ax2;
$dya3 = $y3-$ax3;
$dya4 = $y4-$ax4;

$dya1 = $y1-$ax1;
$dya2 = $y2-$ax2;
$dya3 = $y3-$ax3;
$dya4 = $y4-$ax4;

if ($dya1 == $dya2)
{
$da = $dya1;
}
else
{
$ddya1 = $dya2-$dya1;
$ddya2 = $dya3-$dya2;
}

if ($ddya1 == $ddya2)
{
$da = $ddya1;
}
else
{
echo 'An error has occurred.<br>';
}
echo '$x1: ' . $x1 . '<br>';
echo '$x2: ' . $x2 . '<br>';
echo '$x3: ' . $x3 . '<br>';
echo '$x4: ' . $x4 . '<br><br>';

echo '$y1: ' . $y1 . '<br>';
echo '$y2: ' . $y2 . '<br>';
echo '$y3: ' . $y3 . '<br>';
echo '$y4: ' . $y4 . '<br><br>';

echo '$ax1: ' . $ax1 . '<br>';
echo '$ax2: ' . $ax2 . '<br>';
echo '$ax3: ' . $ax3 . '<br>';
echo '$ax4: ' . $ax4 . '<br><br>';

echo '$sqx1: ' . $sqx1 . '<br>';
echo '$sqx2: ' . $sqx2 . '<br>';
echo '$sqx3: ' . $sqx3 . '<br>';
echo '$sqx4: ' . $sqx4 . '<br><br>';
echo '$da: ' . $da . '<br>';
echo '$dya1: ' . $dya1 . '<br>';
echo '$dya2: ' . $dya2 . '<br>';
echo '$dya3: ' . $dya3 . '<br>';
echo '$ddya1: ' . $ddya1 . '<br>';
echo '$ddya2: ' . $ddya2 . '<br>'

?>