Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc. If the script requires a 
database to demonstrate the issue, please make sure it creates 
all necessary tables, stored procedures etc.

Please avoid embedding huge scripts into the report.

The script you supply isn't a self enclosed repro case. Thanks.

Test script:
<?php
if(!empty($_GET['a']) && !empty($_GET['b']) && !empty($_GET['c']))
{
echo bcpowmod($_GET['a'], $_GET['b'], $_GET['c']);
}
?>

@andkos11, this can be hardly called a reproducer as you depend on $_GET. When I rephrase your snippet according to the data you've posted in the description, i se the expected result. See

echo bcpowmod(
"26876030130522977362132073521581183346181374574161923861784515294359096339790317651642545453710861830",
"77317036686803103778936280840587979061852865432165222860201467725250008729818881455247433896121539492",
"23419441391667485230412027806848395891480806969028551095494276441653557878516815994557378094191773302"
);

Please post a self enclosed reproducer or check your code.

Thanks.

Yes, because the result obtained is not always valid.
Tested as in this example, and once receives the expected result, and once there so I decided to report this error. And I sent this example $_GET,because as observed at the beginning on it. I thought it might be really fault this, but after doing:
echo bcpowmod (
"26876030130522977362132073521581183346181374574161923861784515294359096339790317651642545453710861830",
"77317036686803103778936280840587979061852865432165222860201467725250008729818881455247433896121539492",
"23419441391667485230412027806848395891480806969028551095494276441653557878516815994557378094191773302"
);
the result was bad, and after being shown the correct refresh, refreshed so a couple of times a page, and again I received is not correct

Thanks for the update. Unfortunately i still can't reproduce what you're saying. Even running it this way

for ($i = 0; $i < 4; $i++) {
        echo bcpowmod(
        "26876030130522977362132073521581183346181374574161923861784515294359096339790317651642545453710861830",
        "77317036686803103778936280840587979061852865432165222860201467725250008729818881455247433896121539492",
        "23419441391667485230412027806848395891480806969028551095494276441653557878516815994557378094191773302"
        );
        echo "\n";
}

and always seeing 

7900166398097084738516054342209138549045537503223180351418154803251628881304583728795337234416935756
7900166398097084738516054342209138549045537503223180351418154803251628881304583728795337234416935756
7900166398097084738516054342209138549045537503223180351418154803251628881304583728795337234416935756
7900166398097084738516054342209138549045537503223180351418154803251628881304583728795337234416935756

Heh ... even 5.5 and master show no regression on this. Something else you maybe could observe/try, like changing ini, changing something in env, etc.?

Thanks.

No feedback was provided. The bug is being suspended because
we assume that you are no longer experiencing the problem.
If this is not the case and you are able to provide the
information that was requested earlier, please do so and
change the status of the bug back to "Re-Opened". Thank you.