SVD: remove union type from helper

Not available in php 7.0

Author: Aweptimum

src/LinearAlgebra/Decomposition/SVD.php

@@ -217,7 +217,7 @@ private static function diagonalize(NumericMatrix $S): array
             // Each column should contain 1 non-zero element
             $j = self::isStandardBasisVector($vector);
 
-            if ($j === false) {
+            if ($j === -1) {
                 throw new MatrixException("S Matrix in SVD is not orthogonal:\n" . (string) $S);
             }
 
@@ -263,11 +263,11 @@ private static function diagonalize(NumericMatrix $S): array
      * 
      * @param Vector $v
      * 
-     * @return int|false The index of the non-zero entry or false if either:
+     * @return int The index of the non-zero entry or -1 if either:
      *      1. There are multiple non-zero entries
      *      2. The vector is a zero vector 
      */
-    private static function isStandardBasisVector(Vector $v): int|false
+    private static function isStandardBasisVector(Vector $v): int
     {
         if ($v->l2Norm() === 0) {
             return false;
@@ -282,7 +282,7 @@ private static function isStandardBasisVector(Vector $v): int|false
                 if ($index === -1) {
                     $index = $i;
                 } else { // If we already found a non-zero component, then return false
-                    return false;
+                    return -1;
                 }
             }
         }