PHP implements addition and subtraction verification code

<?php

header(&#39;Content-Type: image/png&#39;);
$im = imagecreatetruecolor( 200 , 50 );

// Create some colors
$white = imagecolorallocate($im, 255, 255, 255);
$grey = imagecolorallocate($im, 128, 128, 128);
$black = imagecolorallocate($im, 0, 0, 0);

imagefilledrectangle($im, 0, 0, 200, 49, $white);

// The text to draw
$rand = rand( 1 ,2 ) ;
$one = rand(1,9);
$two = rand(1,9);
if( $rand == 1 ){

    $result = $one + $two;
    $text = "$one+$two=?";

}else{
    if( $one < $two ){
        list( $two , $one ) = [ $one , $two ];
    }
    $result = $one - $two;
    $text = "$one-$two=?";
}

session_start();
$_SESSION[&#39;vcode&#39;] = $result;
// Replace path by your own font path
$font = &#39;./Arvo-Regular.ttf&#39;;

//imageline( $im , 0 , rand( 1,49 ) , 199 , 30 , $grey );

// Add the text
$i = 0;
$len = strlen( $text );
while( $i < $len  ){
    if( is_numeric( $text[$i] ) ){
        imagettftext($im, 20, rand(-45,45), 20 * ($i+1) , 30, $black, $font, $text[$i]);
    }else{
        imagettftext($im, 20, 0 , 20 * ($i+1) , 30, $black, $font, $text[$i]);
    }
//echo $text[$i];
    $i ++;
}
// Using imagepng() results in clearer text compared with imagejpeg()
imagepng($im);
imagedestroy($im);