First, let me state that print has not been deprecated in favor of echo.  At least I couldn’t find any evidence of it.  (You know who you are.)

There are two clear differences between  print and echo in php.

• print always returns 1, echo does not return anything
  This means that print can be used in expressions, where echo cannot:

  
  if (print "hi" == 1) { print "print returned 1"; }
  
  

• echo concatenates arguments, print does not
  This means you can use commas instead of periods for concatenation:

  
  $enthusiasm = array('.', '!', '!!', '!!!');
  echo "Hello", "," . " ", "world", $enthusiasm[1];
  
  

Note that this means print can produce undesired results in expressions if you expect the parentheses to take precedence, as they do with a function call.  See this comment on the php.net reference documentation.

See also this post about the differences between print and echo:

http://www.faqts.com/knowledge_base/view.phtml/aid/1/fid/40

Also see this comment about performance differences:

http://www.php.net/manual/en/function.print.php#66392

Short answer, the performance difference is negligible.

An interesting experiment might be to compare argument based concatenation in echo with the concatentation operator in print:

<? #hidad.php
ob_start();
for($i=0; $i<$argv[1]; $i++) { echo 'Hi ', 'dad'; }
ob_end_clean();
?>

<? #himom.php
ob_start();
for($i=0; $i<$argv[1]; $i++) { print 'Hi '. 'mom'; }
ob_end_clean();
?>

Results

c:\temp> time php himom.php 1000000
real    0m1.344s
user    0m0.046s
sys     0m0.000s

c:\temp> time php himom.php 1000000
real    0m1.306s
user    0m0.015s
sys     0m0.031s

c:\temp> time php himom.php 1000000
real    0m1.318s
user    0m0.000s
sys     0m0.046s

c:\temp> time php hidad.php 1000000
real    0m1.215s
user    0m0.015s
sys     0m0.031s

c:\temp> time php hidad.php 1000000
real    0m1.180s
user    0m0.000s
sys     0m0.031s

c:\temp> time php hidad.php 1000000
real    0m1.242s
user    0m0.015s
sys     0m0.015s

It looks like there is a slight edge to echo argument concatenation, at least with my limited sample on Windows Vista (using time via Cygwin for benchmarking.)  echo also beat out print with a concatenation operator by a smaller margin, but the differences are too slight to be definitive.

I used output buffering to remove IO from the equation.  I used ob_end_clean() instead of ob_flush() to avoid printing out the results, though we might still be primarily measuring the  efficiency of deleting the buffer.

I tried a smaller result set (10000 vs. 1000000) and got comparable measurements, of about 1/3 the duration, which I’d guess means that most of that time is spent initializing the php interpreter.

c:\temp> time php hidad.php 10000
real    0m0.337s
user    0m0.015s
sys     0m0.047s

c:\temp> time php himom.php 10000
real    0m0.342s
user    0m0.000s
sys     0m0.062s

c:\temp> time php himom.php
real    0m0.315s
user    0m0.031s
sys     0m0.015s

c:\temp> time php hidad.php
real    0m0.330s
user    0m0.000s
sys     0m0.046s

Also note that I modified my script to use a static iteration number (1000000) instead of a variable ($argv[1]) .and got significantly greater speed improvement than the difference between using print and echo or the difference between dot concatenation and comma (argument) concatenation.

<? #himom.php
ob_start();
for($i=0; $i<1000000; $i++) { print 'Hi '. 'mom'; }
ob_end_clean();
?>

c:\temp>time php himom.php
real    0m1.055s
user    0m0.000s
sys     0m0.062s

c:\temp>time php hidad.php
real    0m0.969s
user    0m0.015s
sys     0m0.015s

Finally, even the difference between printing the text “Hi mom” and “Hi dad” appears slightly different, but now we’re just getting lost in noise.