Sorry, but your problem does not imply a bug in PHP itself.  For a
list of more appropriate places to ask for help using PHP, please
visit http://www.php.net/support.php as this bug system is not the
appropriate forum for asking support questions.  Due to the volume
of reports we can not explain in detail here why your report is not
a bug.  The support channels will be able to provide an explanation
for you.

Thank you for your interest in PHP.

This IS a bug.
http://de.php.net/operators lists unary - having higher precedence than the arithmetic operators, and that is the way it should be. So, this statement should evaluate to 1/(-2)*5 and that is -2.5 and not -0.1.

<?php

echo 1 / -2 * 5;   // -0.1
$a = 2;
echo 1 / -$a * 5;  // -0.1
$a = -2;
echo 1 / $a * 5;   // -2.5
echo 1 / -$a * 5;  // 0.1

?>

Looks like a problem with the parsing of unary negation.

Unary negation in PHP is basically 0-value (i.e. it's subtraction). The docs and the code tell difference things - the docs say unary and binary minus have different priority but in the code it's the same.

This bug has been fixed in CVS.

Snapshots of the sources are packaged every three hours; this change
will be in the next snapshot. You can grab the snapshot at
http://snaps.php.net/.
 
Thank you for the report, and for helping us make PHP better.

Fixed in 5/6, not in 4 though (due to objection from Derick).