Just because PHP allows you to do something, doesn't mean that it is the best thing to do.  For example, PHP will automatically convert single word strings (non-quote/apostrophe delimited) into an actual string if required.

$arr = array();

for($x=0;$x<1000000;$x++)
{
    $arr[foo]='bar';
}

In this case, PHP will automatically convert foo to the string 'foo'. However, this up conversion doesn't come without a cost. For example, when timing the use of this script, the following are the results:

$ time php test.php 

real    0m1.641s
user    0m1.424s
sys     0m0.044s

However, when running the following script and not forcing it to up convert, the results are extremely different.

$arr = array();

for($x=0;$x<1000000;$x++ )
{
    $arr['foo']='bar';
}

In this case, the word 'foo' is already pre-defined as a string, so no up conversion is required. The time for this is as follows:

$ time php test.php 

real    0m0.467s
user    0m0.292s
sys     0m0.052s

As you can see, by including the quotes and telling PHP that it actually is a string, you can potentially reduce the execution time for your PHP scripts.

Keep this in mind when using the associative arrays in PHP.