Christian's code works well, but if you want to be able to hide the user input and not echo it to screen, you need to add -s to the read command. The code below is an expanded function that allows an optional prompt and optional hiding of the input:
function read_password($prompt=null, $hide=false)
{
if($prompt) print $prompt;
$s = ($hide) ? '-s' : '';
$f=popen("read $s; echo \$REPLY","r");
$input=fgets($f,100);
pclose($f);
if($hide) print "\n";
return $input;
}