want to know something about xml [duplicate]

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• What is the advantage of using Heredoc in PHP ? [closed] 5 answers

i am learning xml and using xml with php.i am little confusing about a operator(<<<) which is used in program here the program

   <?php
$xmldata = <<<data
<songs>
   <song dateplayed="2011-07-24 19:40:26">
    <title>I left my heart on Europa</title>
    <artist>Ship of Nomads</artist>
</song>
<song dateplayed="2011-07-24 19:27:42">
    <title>Oh Ganymede</title>
    <artist>Beefachanga</artist>
</song>
<song dateplayed="2011-07-24 19:23:50">
    <title>Kallichore</title>
    <artist>Jewitt K. Sheppard</artist>
</song>
</songs>
data;

?>

why i need <<< after equals(Xmldata= <<