strlen versus isset($string{14})

stringtest.php

<?php
    $a = <<<EOT
hkjahkdjhakdjhask jdha ksjdhakjsdhkajhsdkjahs dkajh sdkajhs dkah sdkajh sdoqi oegpwgpsgsldjglsj dlkjs lgkjs ldkgjls
lsk dglks gljs ldgjsldjg lsdjg lsjg lskjglksdjglskjdglksdjg lskdjglksjdglksj dfsf lskdjfklsjdf lksjdflkjslkjkd dkdk
EOT;

$t = microtime(true);

for ($i = 0; $i < 1000000; $i++) {
    $sl = isset($a{14});
}    

$t1 = microtime(true);

echo $t1 - $t . chr(10);

$t = microtime(true);

for ($i = 0; $i < 1000000; $i++) {
    $sl = strlen($a);
}    

$t1 = microtime(true);

echo $t1 - $t;

// 0.048465967178345
// 0.12918090820312