Use single quoted strings unless you need to interpolate variables into your string. This saves php the time to scan the string for contained variables and saves about 50% execution time.

data
double quotes: 0.001505970954895
single quotes: 0.00078308582305908
difference...: 0.00072288513183594

benchmark
function getmicrotime($t) {
list($usec, $sec) = explode(" ",$t);
return ((float)$usec + (float)$sec);
}
#
$start = microtime();
$a = array();
for($i=0;$i<100;$i++) {
array_push($a, "Aaron rules!");
}
$end = microtime();
$t1 = (getmicrotime($end) - getmicrotime($start));
echo "<pre>double quotes: $t1<br>";
#
unset($a);
#
$start = microtime();
$b = array();
for($i=0;$i<100;$i++) {
array_push($b, 'Aaron rules!');
}
$end = microtime();
$t2 = (getmicrotime($end) - getmicrotime($start));
$t3 = $t1-$t2;
echo "single quotes: $t2<br>difference...: $t3</pre>";

__________________

J.Vijayanand

Use concatenation rather than sprintf(). Use sprintf rather than interpolation.
Use sprintf to build more complex strings such as SQL query strings.

data
concatenation: 0.0008620023727417
sprintf......: 0.0017820596694946
interpolate..: 0.002047061920166

benchmark
$x = 'Aaron';
$y = 'C.';
$start = microtime();
for($i=0;$i<100;$i++) {
$a = $x . $y . ' rules!<br/>';
}
$end = microtime();
$t1 = (getmicrotime($end) - getmicrotime($start));
echo "<pre>concatenation: $t1<br>";
#
unset($a);
#
$start = microtime();
for($i=0;$i<100;$i++) {
$a = sprintf('%s %s rules!<br/>', $x, $y);
}
$end = microtime();
$t1 = (getmicrotime($end) - getmicrotime($start));
echo "<pre>sprintf......: $t1<br>";
#
unset($a);
#
$start = microtime();
for($i=0;$i<100;$i++) {
$a = "$x $y rules!<br/>";
}
$end = microtime();
$t2 = (getmicrotime($end) - getmicrotime($start));
$t3 = $t1-$t2;
echo "interpolate..: $t2<br>difference...: $t3</pre>";

__________________

J.Vijayanand

Use push/join rather than concatenation.

data
using concat: 0.00052297115325928 seconds
using array.: 0.00033998489379883 seconds
difference..: 0.00018298625946045 seconds
benchmark
$x = array('Taylor','Zac','Isaac','Leo','Aaron','Nick', 'Scott',
'Dave','Bob','Clint','Chris','Thomas','Ryan','Denn is',
'Thimo','Steve');
#
$s1 = microtime();
foreach($x as $val) {
$s .= $val . ', ';
}
$e1 = microtime();
#
unset($s);
$s = array();
#
$s2 = microtime();
foreach($x as $val) {
array_push($s, $val);
}
$y = implode(', ', $s);
$e2 = microtime();
#
$t1 = (getmicrotime($e1) - getmicrotime($s1));
$t2 = (getmicrotime($e2) - getmicrotime($s2));
$td = $t1 - $t2;
#
echo "<pre>using concat: $t1 seconds<br>
using array.: $t2 seconds<br><br>
difference..: $td seconds</pre>";

__________________

J.Vijayanand

For outputting text with variables, use the little-known fact that echo allows several arguments, instead of concatenation or double quotes (which are twice as slow!):

echo 'text', $var, 'more text', $anothervar;

data:
commas.......: 0.0024470090866089
concatenation: 0.0028690099716187
double quotes: 0.0044499635696411

script:
<?
$x = 'H.'; $y = 'J.'; $z = 'O.';
$start = microtime();
for($i=0;$i<100;$i++) { echo $x, ' ', $y, ' ', $z, ' rules! '; }
$end = microtime();
$t1 = (getmicrotime($end) - getmicrotime($start));
unset($a);
#
$start = microtime();
for($i=0;$i<100;$i++) { echo $x.' '.$y.' '.$z.' rules! '; }
$end = microtime();
$t2 = (getmicrotime($end) - getmicrotime($start));
unset($a);
#
$start = microtime();
for($i=0;$i<100;$i++) { echo "$x $y $z rules! "; }
$end = microtime();
$t3 = (getmicrotime($end) - getmicrotime($start));
unset($a);
#
echo "<pre>commas.......: $t1<br></pre>";
echo "<pre>concatenation: $t2<br></pre>";
echo "<pre>double quotes: $t3</pre>";
function getmicrotime($t) {
list($usec, $sec) = explode(" ",$t);
return ((float)$usec + (float)$sec);
}
?>

__________________

J.Vijayanand

Use PHP´s feature to easily switch between html and php mode whenever you want/need to. This comes at very little cost. When you need to output just a single variable use <?=$var?> instead of <?echo $var?>.
The printf() approach was faster once more than 8 variables were used to build the option elements.

data
html/php mode (<?=)...: 0.2423449754715
html/php mode (<?echo): 0.25177192687988
printf()..............: 0.27242302894592

__________________

J.Vijayanand