3v4l.org - online PHP shell, test in 80+ different PHP versions!

<?php
function test( $input) { 
 $doc = new DOMDocument;
	$doc->loadHTML( $input);
	$p = $doc->getElementsByTagName('p')->item(0);
	$children = $p->childNodes;
 $img_found = false; $error = false;
 foreach( $children as $child) {
 if( $child->nodeType == XML_TEXT_NODE && !$img_found) {
 // Text before the image, OK, continue on
 continue;
 }
 if( $child->nodeType == XML_ELEMENT_NODE) {
 if( !($child->tagName == 'img')) {
 // echo "Invalid HTML element found";
 $error = true;
 } else {
 $img_found = true;
 }
 } else {
 // echo "Invalid node type!";
 $error = true;
 }
 }
 if( $img_found && !$error) {
 echo 'ok' . "\n";
 } else {
 echo 'NOT ok' . "\n";
 }
}

$input = '<img src="#" />'; // ok
test( $input);

$input = '<img src="#" /><img src="#" />'; // ok
test( $input);

$input = 'xxx<img src="#" />'; // ok
test( $input);

$input = '<img src="#" />xxx'; // NOT ok
test( $input);

based on aavYc

Output for 5.0.0 - 5.5.0beta2: ok ok ok NOT ok
Output for 4.4.2 - 4.4.9: Parse error: syntax error, unexpected T_OBJECT_OPERATOR in /in/r942U on line 5
Process exited with code 255.
Output for 4.3.2 - 4.3.4: Parse error: parse error in /in/r942U on line 5
Process exited with code 255.
Output for 4.3.0 - 4.3.1, 4.3.5 - 4.4.1: Parse error: parse error, unexpected T_OBJECT_OPERATOR in /in/r942U on line 5
Process exited with code 255.