PHP speed tests `json_encode` vs `serialize`

speedtest.php

<?php

ini_set( 'display_errors', 1 );
error_reporting( E_ALL );

//  Make a bit, honkin test array
//  You may need to adjust this depth to avoid memory limit errors
$testArray = fillArray( 0, 5 );

//  Time json encoding
$start = microtime( true );
json_encode( $testArray );
$jsonTime = microtime( true ) - $start;
echo "JSON encoded in $jsonTime seconds<br>";

//  Time serialization
$start = microtime( true );
serialize( $testArray );
$serializeTime = microtime( true ) - $start;
echo "PHP serialized in $serializeTime seconds<br>";

//  Compare them
if ( $jsonTime < $serializeTime )
{
    echo "json_encode() was roughly " . number_format( ($serializeTime / $jsonTime - 1 ) * 100, 2 ) . "% faster than serialize()";
}
else if ( $serializeTime < $jsonTime )
{
    echo "serialize() was roughly " . number_format( ($jsonTime / $serializeTime - 1 ) * 100, 2 ) . "% faster than json_encode()";
} else {
    echo 'Unpossible!';
}

function fillArray( $depth, $max )
{
    static $seed;
    if ( is_null( $seed ) )
    {
        $seed = array( 'a', 2, 'c', 4, 'e', 6, 'g', 8, 'i', 10 );
    }
    if ( $depth < $max )
    {
        $node = array();
        foreach ( $seed as $key )
        {
            $node[$key] = fillArray( $depth + 1, $max );
        }
        return $node;
    }
    return 'empty';
}