<?php

$ch = curl_init('https://i2.wp.com/www.fatosmorina.com/wp-content/uploads/2018/11/hello-world.png?ssl=1');
$fp = fopen('placa.png', 'wb');
curl_setopt($ch, CURLOPT_FILE, $fp);
curl_setopt($ch, CURLOPT_HEADER, 0);
curl_exec($ch);
curl_close($ch);
fclose($fp);

$im = imagecreatefrompng('placa.png');

if($im && imagefilter($im, IMG_FILTER_GRAYSCALE))
{
    echo 'Image converted to grayscale.';

    imagepng($im, 'placa_convertida.png');
}
else
{
    echo 'Conversion to grayscale failed.';
}

imagedestroy($im);

?>