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About the optimizations:
About the warnings:
According to Google and my independent research, single quotes--especially when concatenating--are quicker than double quotes. If this option is enabled, all double quotes will be changed into single quotes in php echo statements.

Here are some examples:
Original: echo "This statement $has a variable in it."
New: echo 'This statement '.$has.' a variable in it.'

Original: echo "This statement $has['an'] array in it."
New: echo 'This statement '.$has['an'].' array in it.'

Original: echo "This statement {$has['an']} array in it."
New: echo 'This statement '.$has['an'].' array in it.'

Original: echo "This statement ${has['an']} array in it."
New: echo 'This statement '.$has['an'].' array in it.'

Original: echo "This statement $has->an object in it."
New: echo 'This statement '.$has->an.' object in it.'

Original: echo "This statement {$has->an['object']->in} it."
New: echo 'This statement '.$has->an['object']->in.' it.'

Original: echo "This statement {${has())} a function in it."
New: echo 'This statement '.$has().' a function in it.'

Original: echo "This statement {${has->a())} function in it."
New: echo 'This statement '.$has->a().' function in it.'