Description:
------------
xml_set_processing_instruction_handler manual page says:

--- BEGIN QUOTE
A processing instruction has the following format: 


<?
       target 
       data?>

--- END QUOTE

The truth is:
<?target 
       data?>

There cannot be any space between '<?' and target.

This can be verified:
1. According to the XML 1.0 Specification, and
2. According to how xml_set_processing_instruction_handler() behaves
     
--- BEGIN QUOTED MATERIAL
2.6 Processing Instructions

[Definition: Processing instructions (PIs) allow documents to contain instructions for applications.]
Processing Instructions
[16]   	PI	   ::=   	'<?' PITarget (S (Char* - (Char* '?>' Char*)))? '?>'	
[17]   	PITarget	   ::=   	Name - (('X' | 'x') ('M' | 'm') ('L' | 'l'))	

REFERENCE:
XML 1.0 (Third Edition): http://www.w3.org/TR/REC-xml/#sec-pi


Reproduce code:
---------------
--- test.php
<?php

$q = '?';
$validPI = <<< EOD
<{$q}xml version="1.0"{$q}>
<test> PHP 5.0.2
<{$q}pi 
    DATA 
{$q}>
</test>
EOD;

$invalidPI = <<< EOD
<{$q}xml version="1.0"{$q}>
<test> PHP 5.0.2
<{$q} pi 
    DATA 
{$q}>
</test>
EOD;

$parser = xml_parser_create();
xml_parser_set_option($parser, XML_OPTION_CASE_FOLDING, 0); 
// this doesn't do anything:
xml_parser_set_option($parser, XML_OPTION_SKIP_WHITE, 0); 
xml_set_element_handler($parser, "start_element", "stop_element"); 
xml_set_character_data_handler($parser, "char_data"); 
xml_set_processing_instruction_handler( $parser, "_handlePI");

echo "--- VALID PI in document:\n$validPI\n";
echo "--- Result of parsing:\n";
xml_parse( $parser, $validPI );
echo "\n";
echo "--- INVALID PI in document:\n$invalidPI\n";
echo "--- Result of parsing:\n";
xml_parse( $parser, $invalidPI );

function _handlePI($parser, $target, $data) {
    echo(">>>".$data."<<<");
}

function start_element($parser, $data, $attribs) {
echo $data;
}
function stop_element($parser, $data ) {
echo $data;

}
function char_data($parser, $data ) {
echo $data;
}
?>


Actual result:
--------------
[greg@p3 test]$ php test.php
--- VALID PI in document:
<?xml version="1.0"?>
<test> PHP 5.0.2
<?pi 
    DATA 
?>
</test>
--- Result of parsing:
test PHP 5.0.2
>>>DATA 
<<<
test
--- INVALID PI in document:
<?xml version="1.0"?>
<test> PHP 5.0.2
<? pi 
    DATA 
?>
</test>
--- Result of parsing:
[greg@p3 test]$