The documentation does not mention, but a closing semicolon at the end of the heredoc is actually interpreted as a real semicolon, and as such, sometimes leads to syntax errors.

This works:

<?php
$foo = <<<END
abcd
END;
?>

This does not:

<?php
foo(<<<END
abcd
END;
);
// syntax error, unexpected ';'
?>

Without semicolon, it works fine:

<?php
foo(<<<END
abcd
END
);
?>

<?php
/*
 * PHP 8.5+: You can interpolate arbitrary expressions in double-quoted strings and heredoc,
 * including method calls with arguments.
 */

class C
{
    public function say(string $s): string
    {
        return $s;
    }

    public function run(): void
    {
        $w = 'WORLD';

        $text = <<<TEXT
        HELLO {$this->say($w)}
        TEXT;
        var_dump($text);
    }
}

new C()->run();

// Output:
// string(11) "HELLO WORLD"

In PHP 8.2 using ${var} in strings is deprecated, use {$var} instead:

<?php
$juice = "apple";

// Valid. Explicitly specify the end of the variable name by enclosing it in braces:
echo "He drank some juice made of {$juice}s.";
?>

You can use the complex syntax to put the value of both object properties AND object methods inside a string.  For example...
<?php
class Test {
    public $one = 1;
    public function two() {
        return 2;
    }
}
$test = new Test();
echo "foo {$test->one} bar {$test->two()}";
?>
Will output "foo 1 bar 2".

However, you cannot do this for all values in your namespace.  Class constants and static properties/methods will not work because the complex syntax looks for the '$'.
<?php
class Test {
    const ONE = 1;
}
echo "foo {Test::ONE} bar";
?>
This will output "foo {Test::one} bar".  Constants and static properties require you to break up the string.

easy transparent solution for using constants in the heredoc format:
DEFINE('TEST','TEST STRING');

$const = get_defined_constants();

echo <<<END
{$const['TEST']}
END;

Result:
TEST STRING

md5('240610708') == md5('QNKCDZO')

This comparison is true because both md5() hashes start '0e' so PHP type juggling understands these strings to be scientific notation.  By definition, zero raised to any power is zero.

Here is a possible gotcha related to oddness involved with accessing strings by character past the end of the string:

$string = 'a';

var_dump($string[2]);  // string(0) ""
var_dump($string[7]);  // string(0) ""
$string[7] === '';  // TRUE

It appears that anything past the end of the string gives an empty string..  However, when E_NOTICE is on, the above examples will throw the message:

Notice:  Uninitialized string offset:  N in FILE on line LINE

This message cannot be specifically masked with @$string[7], as is possible when $string itself is unset.

isset($string[7]);  // FALSE
$string[7] === NULL;  // FALSE

Even though it seems like a not-NULL value of type string, it is still considered unset.

<?php 
\\Example # 10 Simple Syntax - Solution for the last "echo" line.

class people {
    public $john = "John Smith";
    public $jane = "Jane Smith";
    public $robert = "Robert Paulsen";

    public $smith = "Smith";
}

$people = new people();

echo "$people->john then said hello to $people->jane.".PHP_EOL;
echo "$people->john's wife greeted $people->robert.".PHP_EOL;
echo "$people->robert greeted the two $people->smiths"; 
\\Won't work 
\\Outputs: Robert Paulsen greeted the two 

/**Solution:**\

echo "$people->robert greeted the two $people->smith\x08s"; 

\\Will work
\\Outputs: Robert Paulsen greeted the two Smiths

?>

Here is an easy hack to allow double-quoted strings and heredocs to contain arbitrary expressions in curly braces syntax, including constants and other function calls:

<?php

// Hack declaration
function _expr($v) { return $v; }
$_expr = '_expr';

// Our playground
define('qwe', 'asd');
define('zxc', 5);

$a=3;
$b=4;

function c($a, $b) { return $a+$b; }

// Usage
echo "pre {$_expr(1+2)} post\n"; // outputs 'pre 3 post'
echo "pre {$_expr(qwe)} post\n"; // outputs 'pre asd post'
echo "pre {$_expr(c($a, $b)+zxc*2)} post\n"; // outputs 'pre 17 post'

// General syntax is {$_expr(...)}
?>

Simple function to create human-readably escaped double-quoted strings for use in source code or when debugging strings with newlines/tabs/etc.

<?php
function doubleQuote($str) {
    $ret = '"';
    for ($i = 0, $l = strlen($str); $i < $l; ++$i) {
        $o = ord($str[$i]);
        if ($o < 31 || $o > 126) {
            switch ($o) {
                case 9: $ret .= '\t'; break;
                case 10: $ret .= '\n'; break;
                case 11: $ret .= '\v'; break;
                case 12: $ret .= '\f'; break;
                case 13: $ret .= '\r'; break;
                default: $ret .= '\x' . str_pad(dechex($o), 2, '0', STR_PAD_LEFT);
            }
        } else {
            switch ($o) {
                case 36: $ret .= '\$'; break;
                case 34: $ret .= '\"'; break;
                case 92: $ret .= '\\\\'; break;
                default: $ret .= $str[$i];
            }
        }
    }
    return $ret . '"';
}
?>

Leading zeroes in strings are (least-surprise) not treated as octal.
Consider:
  $x = "0123"  + 0;   
  $y = 0123 + 0;
  echo "x is $x, y is $y";    //prints  "x is 123, y is 83"
in other words:
 * leading zeros in numeric literals in the source-code are interpreted as "octal", c.f. strtol().
 * leading zeros in strings (eg user-submitted data), when cast (implicitly or explicitly) to integer are ignored, and considered as decimal, c.f. strtod().