First, here is the standard legend with color boxes.

<?php
# PHPlot Example: Linepoints plot with legend using color boxes or shape markers
require_once 'phplot.php';

# The variable $use_shapes can be set to TRUE in another
# script which calls this script, to use shape markers
# rather than color boxes in the legend.
if (empty($use_shapes)) $use_shapes = FALSE;
# The variable $plot_type can be set in another script as well.
if (empty($plot_type)) $plot_type = 'linepoints';

# Use data labels to display only the points we want,
# but specify the same values for X to get the correct
# spacing.
$data = array(
  array('1990', 1990, 41308, 21015, 62634),
  array('1995', 1995, 44310, 13883, 61529),
  array('2000', 2000, 46772,  9000, 59421),
  array('2004', 2004, 46887,  7738, 57754),
  array('2006', 2006, 45441,  6888, 53179),
  array('2008', 2008, 42757,  5840, 49115),
);
$legend_text = array('Morning Papers', 'Evening Papers', 'Sunday Papers');

$plot = new PHPlot(800, 600);
$plot->SetImageBorderType('plain'); // Improves presentation in the manual
$plot->SetTitle("US Daily Newspaper Circulation\n"
             .  $plot_type . ' plot with SetLegendUseShapes('
             . ($use_shapes ? 'True' : 'False') .  ')');
$plot->SetPlotType($plot_type);
$plot->SetDataType('data-data');
$plot->SetDataValues($data);
$plot->SetPlotAreaWorld(1988, 0, 2010, 80000);
$plot->SetYTickIncrement(10000);
$plot->SetLegend($legend_text);
$plot->SetXTickPos('none');
$plot->SetDrawXDataLabelLines(True);
$plot->SetLegendUseShapes($use_shapes); // Use color boxes or shapes
$plot->SetPointSizes(12); // Make points bigger for visibility
$plot->SetLineStyles('solid'); // Make all lines solid
$plot->SetLineWidths(2); // Make all lines thicker

$plot->DrawGraph();

Changing the call to SetLegendUseShapes results in a legend using point shapes instead of color boxes. (This requires PHPlot-5.4.0 or later.)

<?php
# PHPlot Example: Linepoints plot with legend using shape markers
# This sets a variable and calls the script directly above.
$use_shapes = TRUE;
require_once 'legendshape.php';

With a line plot, the shape markers are line segments rather than point markers. (This requires PHPlot-6.0.0 or later.)

<?php
# PHPlot Example: Line plot with legend using line markers
# This sets variables and calls the script directly above.
$use_shapes = TRUE;
$plot_type = 'lines';
require_once 'legendshape.php';