Description:
------------
When executing the strip_tags function, using a string that starts with the lesser then or equal operator, it will be deleted...

I accidentally found this out when stripping tags of an array with values, used for a <select>-element. When I strip the string '<= (Less then or equal)', it will result into an empty string.

I also tried to add it to the $allowable_tags parameter, like this strip_tags('<= (Less then or equal)', '<=>'), but this doesn't seem to work either.

Test script:
---------------
<?php

    $array = [
        '= (Equal)',
        '!= (Not equal)',
        '> (Greater then)',
        '>= (Greater then or equal)',
        '< (Less then)',
        '<= (Less then or equal)',
    ];
              
    foreach( $array as $key => $value )
    {
        echo $key."\t=>\t".strip_tags($value)."\n";
    }

Expected result:
----------------
0	=>	= (Equal)
1	=>	!= (Not equal)
2	=>	> (Greater then)
3	=>	>= (Greater then or equal)
4	=>	< (Less then)
5	=>	<= (Less then or equal)


Actual result:
--------------
0	=>	= (Equal)
1	=>	!= (Not equal)
2	=>	> (Greater then)
3	=>	>= (Greater then or equal)
4	=>	< (Less then)
5	=>