Description:
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I have been through one of the filter_vars() functions and have found some attributes missing from the list...

The documentation states: "Returns TRUE for "1", "true", "on" and "yes"."

However I feel this needs to be extended further to allocate two more attributes to be triggered at "TRUE" there are...

"T" and "Y" (T = true, Y = Yes)

Test script:
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<?php

$str = "1";
var_dump(filter_var($str, FILTER_VALIDATE_BOOLEAN)); // output: true
$str = "yes";
var_dump(filter_var($str, FILTER_VALIDATE_BOOLEAN)); // output: true
$str = "on";
var_dump(filter_var($str, FILTER_VALIDATE_BOOLEAN)); // output: true
$str = true;
var_dump(filter_var($str, FILTER_VALIDATE_BOOLEAN)); // output: true

// missing (potential bug):

$str = "t";
var_dump(filter_var($str, FILTER_VALIDATE_BOOLEAN)); // output: false (should be true, t = true)
$str = "y";
var_dump(filter_var($str, FILTER_VALIDATE_BOOLEAN)); // output: false (should be true, y = true)