very strange strpos() behaviour

Question

I can't find an explanation for this.

I am looking to see if a certain CSS definition exists within a CSS file.

The haystack is $css and the needle is 'tbs_wwrapper'

My haystack DOES contain 'tbs_wrapper' (with one 'w') but does NOT contain 'tbs_wwrapper' (with two 'w') and yet my search function returns true as if there was a match.

How is this possible?

Here is my function:

function tbs_quick_css_find($definition) {
global $css;
if ( strpos(PostFriendlyCSS_ENcode($css), $definition) !== false )
    return true; // match found
else
    echo '<span class="error">The definition "'.PostFriendlyCSS_DEcode($definition).'" was not found in the CSS file</span>';
}