result.txt

Test String: 
< echo "Hello O'Test"; ?>

FILTER_SANITIZE_STRING: 
string(0) ""

FILTER_UNSAFE_RAW with flags: FILTER_FLAG_STRIP_LOW, 
                              FILTER_FLAG_STRIP_HIGH, 
                              FILTER_FLAG_ENCODE_LOW, 
                              FILTER_FLAG_ENCODE_HIGH, 
                              FILTER_FLAG_ENCODE_AMP:
                              
string(25) "< echo "Hello O'Test"; ?>"

test.php

<?php

$strTestString = "< echo \"Hello O'Test\"; ?>";

echo "Test String: ".PHP_EOL;
echo $strTestString.PHP_EOL;

echo PHP_EOL;

echo "FILTER_SANITIZE_STRING: ".PHP_EOL;
echo var_dump(filter_var($strTestString, FILTER_SANITIZE_STRING));

echo PHP_EOL;

echo "FILTER_UNSAFE_RAW: ".PHP_EOL;
echo var_dump(filter_var($strTestString, FILTER_UNSAFE_RAW, array(
        FILTER_FLAG_STRIP_LOW,
        FILTER_FLAG_STRIP_HIGH,
        FILTER_FLAG_ENCODE_LOW,
        FILTER_FLAG_ENCODE_HIGH,
        FILTER_FLAG_ENCODE_AMP
    ))).PHP_EOL;