MDL-59152 mlbackend_php: Check PHP version

Part of MDL-57791 epic.

lib/mlbackend/php/classes/processor.php

@@ -55,6 +55,9 @@ class processor implements \core_analytics\predictor {
     protected $limitedsize = false;
 
     public function is_ready() {
+        if (version_compare(phpversion(), '7.0.0') < 0) {
+            return get_string('errorphp7required', 'mlbackend_php');
+        }
         return true;
     }
 

lib/mlbackend/php/lang/en/mlbackend_php.php

@@ -1,7 +1,8 @@
 <?php
 
-$string['pluginname'] = 'PHP predictor';
+$string['datasetsizelimited'] = 'Only a part of the evaluation dataset has been evaluated due to its size. Set $CFG->mlbackend_php_no_memory_limit if you are confident that your system can cope a {$a} dataset';
 $string['errorcantloadmodel'] = 'Model file {$a} does not exist, ensure the model has been trained before using it to predict.';
-$string['errornotenoughdata'] = 'The evaluation results varied too much, you could try to gather more data to ensure the model is valid. Evaluation results standard deviation = {$a->deviation}, maximum recommended standard deviation = {$a->accepteddeviation}';
 $string['errorlowscore'] = 'The evaluated model prediction accuracy is not very high, some predictions may not be accurate. Model score = {$a->score}, minimum score = {$a->minscore}';
-$string['datasetsizelimited'] = 'Only a part of the evaluation dataset has been evaluated due to its size. Set $CFG->mlbackend_php_no_memory_limit if you are confident that your system can cope a {$a} dataset';
+$string['errornotenoughdata'] = 'The evaluation results varied too much, you could try to gather more data to ensure the model is valid. Evaluation results standard deviation = {$a->deviation}, maximum recommended standard deviation = {$a->accepteddeviation}';
+$string['errorphp7required'] = 'The PHP machine learning backend requires PHP 7';
+$string['pluginname'] = 'PHP machine learning backend';