Running shell_exec('which java') in PHP return nothing

Question

If I run

which java

from the command line I get the proper input (/usr/java/.../bin/java). However if I run it in a php script:

<?
  echo 'java. ' . shell_exec('which java');
  echo 'ls. ' . shell_exec('which ls');
?>

nothing gets printed out for which java but I get the proper results for which ls...