First open Find and Replace functionality with CTRL + R and then check the boxes Match Case and Regex (and if necessary In Selection):

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1. To replace camelCase to snail_case as in in the question:

find: ([A-Z])
replace: _\l$1

someThing -> some_thing

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2. To replace UPPERCASE words to lowercase words use \L

find: (\w*)
replace: \L$1

SOMETHING -> something

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3. To replace lowercase words to UPPERCASE words use \U

find: (\w*)
replace: \U$1

something -> SOMETHING

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4. To replace first character of words with lowercase use \l

find: (\w*)
replace: \l$1

Something -> something

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5. To replace first character of words with UPPERCASE use \u

find: (\w*)
replace: \u$1

something -> Something

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Note: Add some extra boundaries

You get best results by adding some additional boundaries that suit your specific case, for example single ' or double quotes " or line breaks \n

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Regex Documentation

Check for details on additional Regular Expression Syntax the documentation for PHPStorm or WebStorm.