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Each point release of PHP has migration notes related to the previous version.  So for example, migration notes for PHP 7.0 show changes that would need to be made to PHP 5.6 code.  So, if you are moving from PHP 5.5 to 7.13, you would need to review the migration notes for PHP 5.6, 7.0, and 7.1, and so so7.2 and 7.3.  So, how far you are coming determines how far back you need to go.  Here are the links:

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