function check_php_version is questionable in some terms

• check for $p_version == PHP_MIN_VERSION will hardly ever evaluate to
  true in real life (who is running exactly PHP 7.0.0?)
• parameter $p_version is not used in else case,
  this was hardly what was wanted, but did not introduce any bug,
  as phpversion() is used instead of the parameter
• check function_exists( 'version_compare' ) is not necessary as
  • according documentation, version_compare() is available starting
    from PHP 4.1 [1]
  • we use version_compare() at various other places in code without
    checking if it exists

Thus

• remove the function
• replace the single place where it's called by using version_compare()

[1] https://www.php.net/manual/de/function.version-compare.php