X-debugger shows unexpected changes when watching variables in a PHP server-side script - centos

Question

I want to know if the X-debugger and/or Apache web-server could be the cause of a debugging issue that I just started seeing. I'm including the following code to show where the unexpected values occur.
Here is the script code:
// The value of $retrim is FALSE.
// The value of $lowValue is "Smokeymoke".
// The value of $highValue is "Te".
// The value of $k is 1 (ONE).
while( true ) { // The condition of the while loop isn't actually true, but is a detail
// that only adds complexity to this issue so isn't shown here.
$newLowValue = substr( $lowValue, 0, $k ); // When executed $newLowValue is
// "Smokeymoke", but should be "S"!
if( $retrim ) { // The value of $retrim is FALSE, so this if-then block of code
// isn't executed.
$newHighValue = substr( $highValue, 0, $k );
} // End of if( $retrim ) ...
if( $newLowValue !== $newHighValue ) { // However, the value of $newHightValue at
// this point doesn't show "Te", but rather
// "T" (!), but since the two aren't the same,
// even if wrong, causes the while-loop to
// exit.
break;
} // End of if( $newLowValue !== $newHighValue ) ...
++$k;
} // End of while( true ) ...
The while-loop, in the above code, exits with a break statement when $newLowValue is not same as $newHighValue. Otherwise, the loop continues executing.
However, with the values shown, the loop exits because $newLowValue and $newHighValue differ, even though the code didn't execute properly as per the comments in the code above, causing the values of these variables not to be correct.
The attachment shows the X-debugger output that I see when debugging the while-loop, above.
The server information from phpinfo.php -
OS version: Linux (CentOS) 3.10.0-514.16.1.el7.x86_64 #1
SMP Wed Apr 12 15:04:24 UTC 2017 x86_64
Apache version: Apache/2.4.6 (CentOS) OpenSSL/1.0.1e-fips PHP/5.6.30
X-Debugger version: 2.5.3
PHP version: 5.6.30
Symfony 2 version: 2.8.28 - app/dev/debug
The client is running -
PHP Storm version: PhpStorm 2016.3.3 Build #PS-163.13906.21, built on
March 8, 2017
Chrome Browser version: Version 75.0.3770.142 (Official Build) (64-bit)
Windows 10 Home version: 1803 OS build 17134.885

Answer 1

Thank you for your comments. Here is the problem that I found ...
It isn't clear from the data that I showed in my question, but the side-effect of using substr on multi-byte character strings was that sometimes the function returned the original un-truncated string, no matter the value of length parameter. Changing from substr to mb_substr stopped this from happening.
Additionally, the if( retrim ) tests, which were assigned true and false boolean values, would fail and the code block inside the if-then statements would execute even when they shouldn't. When I changed these to if( retrim === true ) along with changing to the mb_substr function, the side-effects stopped occurring.