MDL-57432 lib: unify PHP version checks

* Introduce a new php compatible library which can be used
  early in execution, without Moodle dependencies
* Use it where we previously had hardcoded checks

This was originally suggested in MDL-39007.

admin/cli/install.php

@@ -146,14 +146,9 @@ define('PHPUNIT_TEST', false);
 
 define('IGNORE_COMPONENT_CACHE', true);
 
-// Check that PHP is of a sufficient version
-if (version_compare(phpversion(), "5.6.5") < 0) {
-    $phpversion = phpversion();
-    // do NOT localise - lang strings would not work here and we CAN NOT move it after installib
-    fwrite(STDERR, "Moodle 3.2 or later requires at least PHP 5.6.5 (currently using version $phpversion).\n");
-    fwrite(STDERR, "Please upgrade your server software or install older Moodle version.\n");
-    exit(1);
-}
+// Check that PHP is of a sufficient version as soon as possible.
+require_once(__DIR__.'/../../lib/phpminimumversionlib.php');
+moodle_require_minimum_php_version();
 
 // set up configuration
 global $CFG;