centre text between 2 coords - php

Question

I am trying to centre a line of text, of known width along a line specified as start and end coordinates.
The purpose is to write text around a polygon, so the lines are not always horizontal.
Currently I have the following function which takes the start x and y and the finish x and y of a line and the width of the text (in pixels).
The text will be drawn starting at x1, y1 at the correct angle to follow the line.
To centre this text on the line I have tried to calculate left padding in pixels which should be applied to x1, y1 to move the text the correct amount from its left origin.
The following function is my attempt at modifying the coordinates to implement the above concept. But its not quite right. I end up with text slightly off line, sometimes x is out sometimes y, depends on the face but neither x or y is correct.
private function CenterTextOnLine(&$x1, &$y1, &$x2, &$y2, $width)
{
$distance = $this->getDistance($x1, $y1, $x2, $y2);
//calculate the left padding required in pixels
$padding = ($distance - $width) / 2;
//what factor do we need to alter x1, y1 by?
$factor = ($distance / $padding);
$gradient = ($y2-$y1)/($x2-$x1); //gradient to alter y by
$x1 += abs($x2-$x1) / $factor; //move start x by factor
$y1 += ($gradient / $factor); //add factor of gradient to start y
return;
}
If anyone can see my error, or knows an algorithm for this purpose, I would very much appreciate your input.
Thanks for your time.

Answer 1

I think this should work:
$y1 += ($y2-$y1)/$factor;
$k = $how_much_distance_I_want/sqrt(1+$gradient*$gradient);
$x1 += $k*$gradient;
$y1 -= $k;
return;
While you are at it, I think the abs in
$x1 += abs($x2-$x1) / $factor;
can lead to unexpected results in case ($x2-$x1) is negative (maybe better to leave it without abs), and depending on how php deals with infinities, you might want to have a plan for the case that $factor === 0.