Valid Range 100-900. I found that any value 551 and above would give me bold.(PECL imagick 2, PECL imagick 3)
ImagickDraw::setFontWeight — Define a espessura da fonte
Esta função não está documentada; apenas a lista de argumentos está disponível.
Define a espessura da fonte a ser usada ao fazer anotações com texto.
weight
Exemplo #1 Exemplo de ImagickDraw::setFontWeight()
<?php
function setFontWeight($fillColor, $strokeColor, $backgroundColor) {
    $draw = new \ImagickDraw();
    $draw->setStrokeColor($strokeColor);
    $draw->setFillColor($fillColor);
    $draw->setStrokeWidth(1);
    $draw->setFontSize(36);
    $draw->setFontWeight(100);
    $draw->annotation(50, 50, "Lorem Ipsum!");
    $draw->setFontWeight(200);
    $draw->annotation(50, 100, "Lorem Ipsum!");
    $draw->setFontWeight(400);
    $draw->annotation(50, 150, "Lorem Ipsum!");
    $draw->setFontWeight(800);
    $draw->annotation(50, 200, "Lorem Ipsum!");
    $imagick = new \Imagick();
    $imagick->newImage(500, 500, $backgroundColor);
    $imagick->setImageFormat("png");
    $imagick->drawImage($draw);
    header("Content-Type: image/png");
    echo $imagick->getImageBlob();
}
?>Valid Range 100-900. I found that any value 551 and above would give me bold.