I was trying to do a simple PHP assertion to verify that a string

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Asked: May 24, 20262026-05-24T02:50:28+00:00 2026-05-24T02:50:28+00:00

I was trying to do a simple PHP assertion to verify that a string

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I was trying to do a simple PHP assertion to verify that a string is not empty, but got confusing results.

I wasn’t aware of the string evaluation feature in assert(), but checked the docs and understood that part.

I included the code with the all the test cases, followed by the output, then followed by the questions. When a case fails with syntax error, I just comment it out and repeat the test so the following cases can get executed, and I show the output for each execution.

The code is about locating an XML element using XMLReader, but that just happened to be my code. So here goes the code first:

<?php

$xml = <<< XML
<depts  xmlns:apl="urn:my:ns" >
    <apl:dept>Dept One</apl:dept>
    <apl:dept>Dept Tow</apl:dept>
</depts>
XML;


$elmLocal = 'dept';
$elmUrn = "urn:my:ns";

$xr = new XMLReader;
$xr->XML($xml);

// move to the first desired element node
$found = false;
while ($xr->read()) {
    if ($xr->localName === $elmLocal && $xr->namespaceURI === $elmUrn) {
        $found = true;
        break;
    }
}
if (!$found) {
    exit("---Error--- No element found with given Name/NS.\n");
}



$var = $xr->name;

echo "---------------- Test Results ---------------->\n";

echo "----- var    =={$var}==\n";
echo "----- xr name=={$xr->name}==\n\n";


echo "------ assertion 00 ------>\n";
assert ($xr->name);

echo "------ assertion 01 ------>\n";
assert ($var);

echo "------ assertion 02 ------>\n";
assert ('$xr->name!==""');   

echo "------ assertion 03 ------>\n";
assert ('$var!==""');   

echo "------ assertion 04 ------>\n";
assert ("{$xr->name}!==''");   

echo "------ assertion 05 ------>\n";
assert ("{$var}!==''");   

echo "------ assertion 06 ------>\n";
assert(!empty($xr->name));

echo "------ assertion 07 ------>\n";
assert(!empty($var));

echo "------ assertion 08 ------>\n";
assert('!empty($xr->name)');

echo "------ assertion 09 ------>\n";
assert('!empty($var)');

echo "------ assertion 10 ------>\n";
assert("!empty({$xr->name})");

echo "------ assertion 11 ------>\n";
assert("!empty({$var})");



// do some processing
$domNode = $xr->expand(); // DOMNode XMLReader::expand ([ DOMNode $basenode ] )


?>

And here goes the execution outputs – notice again that there are commenting out on fatal syntax failure and repeat execution, you can just assume that each assertion was tested separately:

#tests#php -v
PHP 5.3.3-7+squeeze3 with Suhosin-Patch (cli) (built: Jun 28 2011 13:13:26)
Copyright (c) 1997-2009 The PHP Group
Zend Engine v2.3.0, Copyright (c) 1998-2010 Zend Technologies
    with Xdebug v2.1.0, Copyright (c) 2002-2010, by Derick Rethans
    with Suhosin v0.9.32.1, Copyright (c) 2007-2010, by SektionEins GmbH


#tests#php -q vars.php
---------------- Test Results ---------------->
----- var    ==apl:dept==
----- xr name==apl:dept==

------ assertion 00 ------>
PHP Parse error:  syntax error, unexpected ':' in /shared/tests/vars.php(40) : assert code on line 1
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:40
PHP Catchable fatal error:  assert(): Failure evaluating code:
apl:dept in /shared/tests/vars.php on line 40
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:40

#tests#php -q vars.php
---------------- Test Results ---------------->
----- var    ==apl:dept==
----- xr name==apl:dept==

------ assertion 01 ------>
PHP Parse error:  syntax error, unexpected ':' in /shared/tests/vars.php(43) : assert code on line 1
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:43
PHP Catchable fatal error:  assert(): Failure evaluating code:
apl:dept in /shared/tests/vars.php on line 43
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:43

#tests#php -q vars.php
---------------- Test Results ---------------->
----- var    ==apl:dept==
----- xr name==apl:dept==

------ assertion 02 ------>
------ assertion 03 ------>
------ assertion 04 ------>
PHP Parse error:  syntax error, unexpected ':' in /shared/tests/vars.php(52) : assert code on line 1
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:52
PHP Catchable fatal error:  assert(): Failure evaluating code:
apl:dept!=='' in /shared/tests/vars.php on line 52
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:52

#tests#php -q vars.php
---------------- Test Results ---------------->
----- var    ==apl:dept==
----- xr name==apl:dept==

------ assertion 02 ------>
------ assertion 03 ------>
------ assertion 05 ------>
PHP Parse error:  syntax error, unexpected ':' in /shared/tests/vars.php(55) : assert code on line 1
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:55
PHP Catchable fatal error:  assert(): Failure evaluating code:
apl:dept!=='' in /shared/tests/vars.php on line 55
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:55

#tests#php -q vars.php
---------------- Test Results ---------------->
----- var    ==apl:dept==
----- xr name==apl:dept==

------ assertion 02 ------>
------ assertion 03 ------>
------ assertion 06 ------>
PHP Warning:  assert(): Assertion failed in /shared/tests/vars.php on line 58
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:58
------ assertion 07 ------>
------ assertion 08 ------>
PHP Warning:  assert(): Assertion "!empty($xr->name)" failed in /shared/tests/vars.php on line 64
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:64
------ assertion 09 ------>
------ assertion 10 ------>
PHP Parse error:  syntax error, unexpected ':', expecting T_PAAMAYIM_NEKUDOTAYIM in /shared/tests/vars.php(70) : assert code on line 1
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:70
PHP Catchable fatal error:  assert(): Failure evaluating code:
!empty(apl:dept) in /shared/tests/vars.php on line 70
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:70

#tests#php -q vars.php
---------------- Test Results ---------------->
----- var    ==apl:dept==
----- xr name==apl:dept==

------ assertion 02 ------>
------ assertion 03 ------>
------ assertion 06 ------>
PHP Warning:  assert(): Assertion failed in /shared/tests/vars.php on line 58
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:58
------ assertion 07 ------>
------ assertion 08 ------>
PHP Warning:  assert(): Assertion "!empty($xr->name)" failed in /shared/tests/vars.php on line 64
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:64
------ assertion 09 ------>
------ assertion 11 ------>
PHP Parse error:  syntax error, unexpected ':', expecting T_PAAMAYIM_NEKUDOTAYIM in /shared/tests/vars.php(73) : assert code on line 1
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:73
PHP Catchable fatal error:  assert(): Failure evaluating code:
!empty(apl:dept) in /shared/tests/vars.php on line 73
PHP Stack trace:
PHP   1. {main}() /shared/tests/vars.php:0
PHP   2. assert() /shared/tests/vars.php:73

Now some comments:

Cases 1,2 are understandable – ie the string evaluation “feature”.

All cases with the assertion expression surrounded by double quotes – figured them out too: variable parsing is done first which results in the syntax error in the execution. So to make case 4 for example work, we can do:

echo "------ assertion 04 ------>\n";
assert ("'{$xr->name}'!==''");

Now the real issue is with cases 6,7. Why is 6 failing while 7 is succeeding. The same with cases 8,9 – 8 failing while 9 succeeding.

Regards.

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1 Answer

Editorial Team · Answer 1 · 2026-05-24T02:50:29+00:00

It’s not about how assert() works but how empty() works.
When used with $object->inaccessibleProp empty first tries the same thing as isset() and if this “test” returns false empty() doesn’t even try to access the element but returns true.
And the XMLReader class obviously doesn’t implement the __isset() method the way you’d need it.

<?php
$xml = <<< XML
<depts  xmlns:apl="urn:my:ns" >
    <apl:dept>Dept One</apl:dept>
    <apl:dept>Dept Tow</apl:dept>
</depts>
XML;

$elmLocal = 'dept';
$elmUrn = "urn:my:ns";

$xr = new XMLReader;
$xr->XML($xml);

// move to the first desired element node
$found = false;
while ($xr->read()) {
    if ($xr->localName === $elmLocal && $xr->namespaceURI === $elmUrn) {
        $found = true;
        break;
    }
}
if (!$found) {
    exit("---Error--- No element found with given Name/NS.\n");
}

$var = $xr->name;
var_dump(isset($xr->name));
var_dump(empty($xr->name));
echo $xr->name;

prints

bool(false)
bool(true)
apl:dept

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