why this is illegal way for declaration php file? [duplicate]

This question already has an answer here:

• Reference — What does this symbol mean in PHP? 18 answers
• What does this symbol mean in PHP <?= 4 answers
• What does '<?=' mean in PHP? 8 answers
• What does <?= mean? 3 answers

I was reading this paper: https://www.exploit-db.com/docs/english/45074-file-upload-restrictions-bypass.pdf
and i saw that i can write a PHP file in this way :

<?=`$_GET['x']`;

so , where is the <?PHP ?>, and this way work for all PHP versions ? or need any conditions ?? .