What's wrong with this XML with PIs (Processing Instructions)?

Question

My XML file does not work correctly now, but worked earlier.

An online validator provided this error:

Message : Invalid '[ "urlset"]' found.

I don't know what is wrong with this urlset. Here is syntax:

<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
    <? foreach ($nizkochastotki AS $data) { ?>
    <url>
        <loc>https://<?=$host?><?=$data['url']?></loc>
        <lastmod><?=date('Y-m-d')?></lastmod>
        <changefreq>weekly</changefreq>
        <priority>0.8</priority>
    </url>
    <? }?>
</urlset>

Answer 1

Without seeing a schema and documentation regarding allowed PIs (<? ... ?> Processing Instructions), we cannot completely "correct" your file, but we can show you what it would take to make it well-formed:

Note that a PI must have a target name the conforms to

[17] PITarget ::= Name - (('X' | 'x') ('M' | 'm') ('L' | 'l'))

[4]  NameStartChar ::= ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] | [#x370-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]
[4a] NameChar ::= NameStartChar | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]
[5]  Name ::= NameStartChar (NameChar)*

that is, among other requirements, it must start with a letter or several of the listed characters.

Note also that a PI target must appear immediately after the <? – no whitespace may intercede:

[16] PI ::= '<?' PITarget (S (Char* - (Char* '?>' Char*)))? '?>'

So, if you remove the space ahead of foreach, and then add whatever PI targets are allowed by the processing application (XXX as a placeholder below), the following XML is now at least well-formed:

<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
    <?foreach ($nizkochastotki AS $data) { ?>
    <url>
        <loc>https://<?XXX =$host?><?XXX  =$data['url']?></loc>
        <lastmod><?XXX =date('Y-m-d')?></lastmod>
        <changefreq>weekly</changefreq>
        <priority>0.8</priority>
    </url>
    <?XXX }?>
</urlset>