(PECL pdflib >= 2.0.0)
PDF_shading — Define blend
$pdfdoc
    , string $shtype
    , float $x0
    , float $y0
    , float $x1
    , float $y1
    , float $c1
    , float $c2
    , float $c3
    , float $c4
    , string $optlist
    )Defines a blend from the current fill color to another color.
This function requires PDF 1.4 or above.