<p>(assume php5) consider</p>

<pre><code>&lt;?php

    $foo = 'some words';

    //case 1
    print "these are $foo";

    //case 2
    print "these are {$foo}";

    //case 3
    print 'these are ' . $foo;
?&gt;
</code></pre>

<p>Is there much of a difference between 1 and 2?</p>

<p>If not, what about between 1/2 and 3?</p>

Well, as with all "What might be faster in real life" questions, you can't beat a real life test.
```
```
function timeFunc($function, $runs)
{
  $times = array();

  for ($i = 0; $i < $runs; $i++)
  {
    $time = microtime();
    call_user_func($function);
    $times[$i] = microtime() - $time;
  }

  return array_sum($times) / $runs;
}

function Method1()
{ 
  $foo = 'some words';
  for ($i = 0; $i < 10000; $i++)
    $t = "these are $foo";
}

function Method2()
{
  $foo = 'some words';
  for ($i = 0; $i < 10000; $i++)
    $t = "these are {$foo}";
}

function Method3()
 {
  $foo = 'some words';
  for ($i = 0; $i < 10000; $i++)
    $t = "these are " . $foo;
}

print timeFunc('Method1', 10) . "\n";
print timeFunc('Method2', 10) . "\n";
print timeFunc('Method3', 10) . "\n";
```
```
Give it a few runs to page everything in, then...
0.0035568
0.0035388
0.0025394
So, as expected, the interpolation are virtually identical (noise level differences, probably due to the extra characters the interpolation engine needs to handle). Straight up concatenation is about 66% of the speed, which is no great shock. The interpolation parser will look, find nothing to do, then finish with a simple internal string concat. Even if the concat were expensive, the interpolator will still have to do it,
after
all the work to parse out the variable and trim/copy up the original string.
Updates By Somnath:
I added Method4() to above real time logic.
```
```
function Method4()
 {
  $foo = 'some words';
  for ($i = 0; $i < 10000; $i++)
    $t = 'these are ' . $foo;
}

print timeFunc('Method4', 10) . "\n";

Results were:

0.0014739
0.0015574
0.0011955
0.001169
```
```
When you are just declaring a string only and no need to parse that string too, then why to confuse PHP debugger to parse. I hope you got my point.