variable echoing

Ok, so one of the threads on here got me thinking the other day, and I want to know the correct way to do things.

Whenever I have a variable that I want to print out in the middle of an echo string, I do the following:

PHP Code:

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echo "I want to print this var: " . $var . " units; 


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And now I've even seen the following on php.net:

PHP Code:

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echo "I want to print this var: $var units"; 


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No quotes, no indication to the processor that it's a php variable other than the dollar sign. Does it matter? Is one slower? And what's the difference between single and double quotes and when should I use either because right now the only time I use single quotes is in SQL queries in the WHERE clause.

That is basicly the power of the double quotes:

PHP Code:

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$a = 'something';

$b1 = "a=$a"; // a=something
$b2 = 'a=$a'; // a=$a

$c['key']='value';

$d1 = "c=$c[key]"; // c=value
$d2 = "c={$c['key']}"; // c=value
$d3 = 'c=$c[key]'; // c=$c[key] 


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Double qoutes expand variables, they are very usefull to make code more readable. There is no significant difference in performance.