Check what really does the coalesce operator

php-coalesce-operator.php

<?php

// undefined variable
echo "?? prints 'default'";
echo "<br>";
var_dump($a ?? 'default');
echo "<br><br>";


echo "?: sends a notice 'undefined variable' and prints 'default'";
echo "<br>";
var_dump($a ?: 'default');
echo "<br><br>";

// empty value
$empty = '';
echo "?? allows a empty string";
echo "<br>";
var_dump($empty ?? 'default');
echo "<br><br>";

echo "?: doesn't";
echo "<br>";
var_dump($empty ?: 'default');
echo "<br><br>";

// false value
$empty = false;
echo "?? allows a false value";
echo "<br>";
var_dump($empty ?? 'default');
echo "<br><br>";

echo "?: doesn't";
echo "<br>";
var_dump($empty ?: 'default');
echo "<br><br>";

// null value
$empty = null;
echo "?? doesn't allow a null value";
echo "<br>";
var_dump($empty ?? 'default');
echo "<br><br>";

echo "?: neither";
echo "<br>";
var_dump($empty ?: 'default');
echo "<br><br>";

// missing array index
$array = ['foo' => 'foo'];
echo "?? allows missing array index";
echo "<br>";
var_dump($array['bar'] ?? 'default');
echo "<br><br>";

echo "?: doesn't and prints a notice";
echo "<br>";
var_dump($array['bar'] ?: 'default');
echo "<br><br>";

The result of the execution is

?? prints 'default'
string(7) "default"

?: sends a notice 'undefined variable' and prints 'default'

NOTICE Undefined variable: a on line number 12
string(7) "default"

?? allows a empty string
string(0) ""

?: doesn't
string(7) "default"

?? allows a false value
bool(false)

?: doesn't
string(7) "default"

?? doesn't allow a null value
string(7) "default"

?: neither
string(7) "default"

?? missing array index
string(7) "default"

?: missing array index

NOTICE Undefined index: bar on line number 60
string(7) "default"