Gradient fills are implemented in SWF as a color ramp, mapped onto a square from (-16384, -16384) to (16384, 16384) TWIPS.
The following code creates a movie, displaying a square with a gradient fill, correctly scaled to the size of the shape.
<?php
//Set the scale to TWIPS rather than PIXELS
ming_setscale(1.0);
//Create the movie
$m = new SWFMovie();
$m->setRate(12.0);
$m->setDimension(400,400);
$m->setBackground(0xff, 0xff, 0xff);
//Width and height of rectangle
$sx = 300;
$sy = 300;
//Create rectangle
$s = new SWFShape();
//Gradient file stuff here...
$g = new SWFGradient();
$g->addEntry(0.0, 0x00, 0x00, 0xff); // Start off Blue
$g->addEntry(1.0, 0xff, 0xff, 0x00); // Finish off Yellow
// Add gradient fill to the shape
$f = $s->addFill($g);
//Centre the gradient fill against the rectangle
$f->moveTo($sx/2, $sy/2);
//Scale it to the shape, so that the entire gradient fits inside the shape
$f->scaleTo($sx/32768, 1);
// Set the shapes fill to be the added gradient fill
$s->setRightFill($f);
//Draw the rectangle
$s->drawLine($sx, 0);
$s->drawLine(0, $sy);
$s->drawLine(-$sx, 0);
$s->drawLine(0, -$sy);
//Add the shape to the movie
$sh = $m->add($s);
$sh->moveTo(50, 50);
// Output the movie
header("Content-type: application/x-shockwave-flash");
$m->output();
?>
More info about this can be obtained from
http://www.openswf.org/spec/SWFfilereference.html#h4FillTypes