diff --git a/BigQuery/composer.json b/BigQuery/composer.json
index 7826449dc1e..855d21c2c6a 100644
--- a/BigQuery/composer.json
+++ b/BigQuery/composer.json
@@ -10,6 +10,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6",
diff --git a/Core/composer.json b/Core/composer.json
index 12474f49cc5..64929b2b4e3 100644
--- a/Core/composer.json
+++ b/Core/composer.json
@@ -16,6 +16,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6",
diff --git a/Datastore/composer.json b/Datastore/composer.json
index 21cf670ee09..c237e68ab52 100644
--- a/Datastore/composer.json
+++ b/Datastore/composer.json
@@ -10,6 +10,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6"
diff --git a/Debugger/composer.json b/Debugger/composer.json
index bddb1b6e560..adf0c449fe5 100644
--- a/Debugger/composer.json
+++ b/Debugger/composer.json
@@ -13,6 +13,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6",
diff --git a/ErrorReporting/composer.json b/ErrorReporting/composer.json
index 0c28fb54a23..6c62dbdda5d 100644
--- a/ErrorReporting/composer.json
+++ b/ErrorReporting/composer.json
@@ -10,6 +10,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "google/cloud-core": "^1.39"
     },
     "suggest": {
diff --git a/Firestore/composer.json b/Firestore/composer.json
index ca638abcdcd..2794d2f2437 100644
--- a/Firestore/composer.json
+++ b/Firestore/composer.json
@@ -12,6 +12,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6"
diff --git a/Language/composer.json b/Language/composer.json
index f56b1d012d6..1548693adfe 100644
--- a/Language/composer.json
+++ b/Language/composer.json
@@ -10,6 +10,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6",
diff --git a/Logging/composer.json b/Logging/composer.json
index 5f785c87f93..bdb36ea0fc1 100644
--- a/Logging/composer.json
+++ b/Logging/composer.json
@@ -10,6 +10,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6",
diff --git a/PubSub/composer.json b/PubSub/composer.json
index c3a3d71391b..7540a76910d 100644
--- a/PubSub/composer.json
+++ b/PubSub/composer.json
@@ -10,6 +10,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6"
diff --git a/Spanner/composer.json b/Spanner/composer.json
index c58d1956d83..89d90ae1382 100644
--- a/Spanner/composer.json
+++ b/Spanner/composer.json
@@ -11,6 +11,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6",
diff --git a/Speech/composer.json b/Speech/composer.json
index aeced47fa3b..02d108b0eb1 100644
--- a/Speech/composer.json
+++ b/Speech/composer.json
@@ -10,6 +10,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6",
diff --git a/Storage/composer.json b/Storage/composer.json
index 26d6bc3776e..bb644655dbc 100644
--- a/Storage/composer.json
+++ b/Storage/composer.json
@@ -10,6 +10,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6",
diff --git a/Trace/composer.json b/Trace/composer.json
index 42431ac6dbf..b0492ee04e6 100644
--- a/Trace/composer.json
+++ b/Trace/composer.json
@@ -11,6 +11,7 @@
      "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6"
diff --git a/Translate/composer.json b/Translate/composer.json
index d55ffb97391..062e3519853 100644
--- a/Translate/composer.json
+++ b/Translate/composer.json
@@ -10,6 +10,7 @@
      "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6"
diff --git a/Vision/composer.json b/Vision/composer.json
index cc8ca5dbeb9..5d296016345 100644
--- a/Vision/composer.json
+++ b/Vision/composer.json
@@ -10,6 +10,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "2.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "erusev/parsedown": "^1.6",
diff --git a/composer.json b/composer.json
index 294386ed4ed..5fceae747a1 100644
--- a/composer.json
+++ b/composer.json
@@ -58,6 +58,7 @@
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0|^8.0",
         "yoast/phpunit-polyfills": "^1.0",
+        "phpspec/prophecy": "^1.10.3",
         "squizlabs/php_codesniffer": "3.*",
         "phpdocumentor/reflection": "^3.0||^4.0",
         "symfony/console": "^3.0",