<p>I mistakenly missed this proposal when submitting my own for a boolean filter.</p>

<p>Firstly, I suggest renaming to Zend_Filter_Bool for naming consistency with Zend_Filter_Int.</p>

<p>Below is modified text from the "theory of operation" from my proposal. I think it is important to roll the following functionality into your version, which primarily addresses only localization issues. Additionally, I hope the below will argue the case for <strong>not</strong> conforming to the way that PHP casting works:</p>

<p>The <code>(bool)</code> and <code>(boolean)</code> casts in PHP consider <code>NULL</code> and empty strings to be <br />
<code>FALSE</code>, and any value that is not specifically <code>FALSE</code> to be <code>TRUE</code>. When filtering data from forms or databases, such assumptions are likely to result in logic errors.</p>

<p>A different set of needs is addressed by filtering as opposed to casting to boolean:</p>
<ul>
<li>It is not always appropriate to assume intent regarding empty or unset input. If a default value is desired, it should be defined by setting the <a href="http://framework.zend.com/manual/en/zend.filter.input.html#zend.filter.input.metacommands.default">Zend_Filter_Input::DEFAULT_VALUE</a> metacommand, or by checking/validating for a <code>NULL</code> value from the results of this filter.</li>
<li>It is helpful to filter for valid boolean literals used by some databases that would not be recognized when casting.</li>
</ul>

<p>The following values should be considered <code>FALSE</code>:</p>
<ul>
<li>the boolean <code>FALSE</code> itself</li>
<li>numeric values that equal 0 such as 0 or 0.0</li>
<li>numeric strings that can be cast to the integer 0 such as "0.0" or "00"</li>
<li>the string "false" (any capitalization allowed)</li>
<li>the string "f" (any capitalization allowed)</li>
<li>the string "no" (any capitalization allowed)</li>
<li>the string "n" (any capitalization allowed)</li>
<li>the string "off" (any capitalization allowed)</li>
</ul>

<p>The following values should be considered <code>TRUE</code>:</p>
<ul>
<li>the boolean <code>TRUE</code> itself</li>
<li>numeric values that equal 1 such as 1 or 1.0</li>
<li>numeric strings that can be cast to the integer 1 such as "1.0" or "01"</li>
<li>the string "true" (any capitalization allowed)</li>
<li>the string "t" (any capitalization allowed)</li>
<li>the string "yes" (any capitalization allowed)</li>
<li>the string "y" (any capitalization allowed)</li>
<li>the string "on" (any capitalization allowed)</li>
</ul>

<p>The filter should output <code>NULL</code> upon attempting to process any value not listed above. </p>