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Case-mapping or case conversion is performed everytime a character is changed from upper case to lower case, or from lower case to upper case. Case converter performs case-mapping everytime you use it.

There are two kind of case-mapping:

  1. Simple case-mapping
  2. Full case-mapping

Simple case-mapping is one-to-one character mapping, for example a single character "A" is replaced with another single character "a".

As you can image, Full case-mapping performs one-to-many character replacements (more precisely one-to-many code-points). In real world use-cases, it's rare to perform full case-mapping, this is because it only concerns a very small set of characters. For example in german language, the letter "ß" is strictly lowercase and should be mapped to "SS" in uppercase words.

Case-Converter behaviour

By default, Case-Converter will perform full case-mapping

// Full case-mapping
$ger = new Convert('Straße');
echo $ger->toUpper(); // output: STRASSE

If you want to perform simple case-mapping then you have to call ->forceSimpleCaseMapping():

// Simple case-mapping
$ger = new Convert('Straße');
echo $ger->toUpper(); // output: STRAßE

As you can see, in full case-mapping string length can change.

Case-Mapping in PHP

PHP 7.3 introduced full case-mapping, you can have one-to-many character mapping. In practice this means than you can have different results depending on your PHP version.

Internally Case-Converter uses mb_convert_case() . This function works in conjunction with specific constants to tell what action to perform. For example:

mb_convert_case('Foo', MB_CASE_UPPER); // FOO

Prior to PHP 7.3, these were the available constants and their use:

Constant Meaning
MB_CASE_UPPER Performs simple upper-case fold conversion.
MB_CASE_LOWER Performs simple lower-case fold conversion.
MB_CASE_TITLE Performs simple title-case fold conversion.

But from PHP 7.3, new constants were added and their meaning changed:

Constant Meaning
MB_CASE_UPPER Performs a full upper-case folding.
MB_CASE_LOWER Performs a full lower-case folding.
MB_CASE_TITLE Performs a full title-case conversion.
MB_CASE_UPPER_SIMPLE Performs simple upper-case fold conversion.
MB_CASE_LOWER_SIMPLE Performs simple lower-case fold conversion.
MB_CASE_TITLE_SIMPLE Performs simple title-case fold conversion.

Locale dependent mapping

Some case-mapping are locale dependent. This is the case of Turkish where the small letter "i" should be replaced by a capital letter with a dot "İ". However, according to documentation:

Only unconditional, language agnostic full case-mapping is performed.

This means that locale dependent mapping are ignored and not performed.


PHP 7.3 Full Case-Mapping and Case-Folding Support
mbstring constant