You can do the entire thing in one line using regex:
$value = preg_replace('/(0)\.(\d+) (\d+)/', '$3$1$2', microtime());
Example:
<?php
$microtime = microtime();
var_dump( $microtime );
var_dump( preg_replace('/(0)\.(\d+) (\d+)/', '$3$1$2', $microtime) );
?>
Output:
string(21) "0.49323800 1385734417"
string(19) "1385734417049323800"