PHP - '...' กับ "..."

        $a = 123;
      
        echo '1 a = $a ' . '<br>';
        echo "2 a = $a " . "<br>";
        echo "3 a = $a นะจ๊ะ " . "<br>";
        echo "4 a = $a " . 'นะจ๊ะ' . '<br>';
        echo "5 a = $aนะจ๊ะ "  . '<br>';
        echo '6 a = $aนะจ๊ะ';

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